Wiki

There are many technical terms you might come across as you learn about repair, how things work, and electrical and electronic fundamentals, but don't let that put you off. Here you will find simple definitions of most of them.

Summary

A primary aim of the Restart Wiki is to assume no previous knowledge, only a willingness to learn, and so we try to introduce and explain technical terms as we go along. But you might dip in and out or read the end of an article before the beginning, or you might consult other sources containing unfamiliar terms.

So this page gathers together as many technical terms as possible that you might come across in your fixing-related research, cross-referenced and linked in to main wiki articles where relevant.

We have an ambition to translate the whole Wiki into several other languages. If you are proficient in a language other than English it would be a great help if you could submit translations of some of these definitions in this Google Sheet.

If you are looking for technical terms in other languages, iFixit have a multilingual dictionary. Whereas the intention of our Glossary is to help you understand unfamiliar terms, the purpose of theirs is to facilitate the translation of repair guides, hence the coverage is somewhat different.

Glossary of Terms

0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Term	Meaning
0 - 9
0-Day	A zero-day is a vulnerability that is found and already being exploited by malicious actors before any fix is available, giving potential victims no time at all (zero days) in which to protect themselves. Also known as an 0-Day (pronounced "oh-day").
2FA	Two Factor Authentication (2FA) and Multi Factor Authentication (MFA) are authentication schemes requiring a user to pass two or more tests, different in kind, in order to prove their identity. These are usually chosen from something you know (for example a password), something you have (a physical token) and something you are (a biometric such as a fingerprint or facial scan). Such a sytem is much harder to compromise than a simple password login.
3 Phase	Three phase is a system of power transmission in which three wires carry AC phased like three blades of a propeller. Consequently they balance out and there is no need for any wires to carry the return current. As a result, three wires can carry three times as much power as two wires in a conventional single phase circuit.
3-Terminal Regulator	A 3-terminal regulator is a device which takes an unregulated input voltage and produces a constant well-defined output voltage. It has just 3 connections (or terminals): the input, the output and a common ground. The 78/79 series integrated circuits are a very common example. 78 devices take positive input and output whereas 79 series take negative. Two further digits in the type code indicate the voltage, for example 7805 produces +5V output.
A
AC	AC or Alternating Current is an electrical supply which reverses direction many times per second. See AC and DC in the Wiki.
AC/DC motor	An AC/DC or Universal motor is a type of electric motor that runs equally well on AC or DC. See DC and Universal (AC/DC) Motors in the Wiki.
Adware	Adware is a type of malware which presents you with unwanted advertising that you never knowingly agreed to receive.
AF	AF or Audio Frequency is a frequency within the audible range of approximately 20Hz - 20kHz.
Aftermarket	Aftermarket refers to spare parts, consumables or accessories for an item not manufactured and sold by the item's manufacturer, but by a third party.
AHCI	AHCI (Advanced Host Controller Interface) is the logical access method used by an operating system to access SATA drives to exploit their capabilities to the full. It supersedes Legacy or IDE mode.
ALU	An ALU or Arithmetic and Logical Unit is that part of a CPU that actually performs arithmetic and logical operations, as commanded by and on data supplied to it by executive functions within the CPU. The 74181 is an early example of a 4-bit ALU implemented as a single silicon chip.
AM	AM or Amplitude Modulation is a method of modulation in which the amplitude of an RF signal is modulated by the AF signal to be transmitted. See Amplitude Modulation in the Wiki.
AMD64	AMD64 is a 64 bit CPU architecture created by AMD as an evolutionary development of the Intel x86 32 bit architecture.
Amp	The Amp is the unit of electric current, i.e. the quantity of electricity passing, a bit like the amount of water flowing in a river. Named after French mathematician and physicist André-Marie Ampère, remembered for his early work on electromagnetism. See Electric circuits, volts amps watts and ohms in the Wiki.
Analogue	Analogue is a method of representing a signal by a voltage (or a current) which is directly proportional to the value of the signal at any instant. For example, the output of a microphone is a voltage which continuously varies in direct proportion to the pressure of the incident sound wave.
Anode	An Anode is a positive electrode.
ARM	ARM, or Advanced RISC Machines is a computer architeture designed for low cost and low power consumption. It is used in huge numbers of smartphones but equally, supercomputers have been built using large swarms of them. It was first introduced in 1985 by Acorn Computers (also known for their BBC Micro) but it has since gone through several generations. The design is owned by Arm Ltd who licence it to other firms to actually manufacture the chips.
ASCII	ASCII (American Standard Code for Information Interchange) is a standard used for representing text in computers and other devices. It uses a set of 128 characters, including letters, numbers, punctuation marks, and control characters, each assigned a unique 7-bit binary number.
ASIC	An Application-Specific Integrated Circuit or ASIC is an integrated circuit designed with a very specific application in mind, unlike most which are designed to be as generally applicable as possible in order to leverage economies of scale. Often the design will be built largely out of a library of standard logic blocks, such as a CPU core, RAM and various peripheral interfacing blocks.
Assembler	Assembly code or assembler is a direct one-to-one representation of the fundamental binary instructions used by a computer, in human-readable form. Instruction codes are written as mnemonics such as ADD and MOV, and storage locations holding the data are given names chosen by the programmer.
Asymmetric cryptography	Public key or asymmetric cryptography is a form of encryption using separate keys for encryption and decryption, neither of which can be derived from the other. Anyone can use the public key to encrypt a message which only the holder of the corresponding private key can decrypt. Alternatively, the holder of a private key can "digitally sign" a message by using it to encrypt a digest of the message, sent with the message itself. A recipient can prove the authorship of the message by successfully decrypting the digest using the author's previously published public key. Public key cryptography is computationally expensive so is often used simply to agree a symmetric encryption key.
ATA	ATA or AT Attachment is an interface standard primarily used for the connection of storage devices and which has gone through a number of iterations, starting with the original IBM PC/AT. Earlier versions used a parallel interface and are now known as PATA but this has now been superseded by a serial version known as SATA.
AWG	AWG or American Wire Gauge is a standard for specifying wire diameters, mainly used in North America. In the UK, SWG (Standard Wire Gauge) is more commonly used.
B
B7G	B7G was a 7-pin base used on miniature all-glass valves.
B9A	B9A was a 9-pin base used on miniature all-glass valves.
Back EMF	Any electric motor functions equally as a dynamo, and when it's running, the dynamo effect generates a voltage known as the back EMF. This always opposes the voltage applied to drive the motor. The electrical work done in opposing the back EMF is what turns up as mechanical work on the motor spindle.
Ballast	Whereas a conventional resistor only draws a current equal to the voltage divided by the resistance, in the case of a LED or a lamp utilising an electrical discharge through a gas or vapour (such as a fluorescent tube) the current increases rapidly once a certain voltage is reached. The current must therefore be limited by the circuit driving it. There are several ways this can be achieved but they are all referred to as a "ballast".
Balun	A balun (short for "balanced to unbalanced") is an electrical device that converts between a balanced signal (one with two conductors carrying equal and opposite signals) and an unbalanced signal (one with a single conductor and a ground). Baluns are commonly used in radio and audio applications to connect balanced transmission lines, such as twisted pair cables, to unbalanced systems, like coaxial cables. They help in matching impedance, reducing interference, and maintaining signal integrity.
Base	The emitter, base and collector are the three terminals of a junction or bipolar transistor. See Transistors in the Wiki.
Bathtub curve	Reliability studies in many fields tend to show a common pattern for failures. Manufacturing defects are the principal cause of early life failures, but once these have all been shaken out the failure rate remains fairly low through the expected lifetime of a device. The failure rate then starts to climb as components begin to wear out. So plotting the failure rate against time gives a U-shaped curve, commonly known as the bathtub curve.
Battery Management System	A BMS or Battery Management System is a circuit designed to control the charging and discharging of a battery, most usually consisting of multiple lithium cells in a battery pack for a laptop, power tool or electric vehicle. Without it, due to the inevitable variation between cells, some cells might be over-charged with potentially dangerous consequences before others have received their full charge, and some might be over-discharged causing permanent damage while others have charge remaining.
BC	BC or Bayonet Cap is a type of lamp base fitted with a push-and-twist action. There are several sizes but the 22mm diameter size is much the commonest. See Types of bulb in the Wiki.
BGA	BGA or Ball Grid Array is a type of surface mount integrated circuit package having connections in an array on the entire underside. Tiny balls of solder attach it to an identcal pattern of connections on the circuit board. This allows for a very large number of connections, not limited to the edges of the package. It's commonly used for processor and memory chips.
Bimetallic Strip	A bimetallic strip consists of two strips of different metals fixed together. When heated up, one strip expands more than the other, causing the combined strip to bend away from the one which is expanding more. Often made into a snap action to create a switch which snaps open at a given temperature.
Binary	Binary is a number system using only the digits 0 and 1, well suited to computers as they can represent those digits by the on and off states of a switch.
BIOS	The BIOS or Basic Input/Output System was a program built in to PCs up until around 2011, which controlled the PC prior to booting and initiated the boot process. (Newer computers use UEFI). See BIOS vs UEFI in the Wiki.
Bipolar Transistor	A bipolar or junction transistor is a type of transistor with 3 semiconductor layers. See Transistors in the Wiki.
Birmingham screwdriver	A Birmingham screwdriver is another name for a hammer. It has been libellously suggested that residents of the fine city of Birmingham don’t know or understand the different use cases for a screwdriver and for a hammer, and prefer to use the latter for all purposes. This is definitely not true, as evidenced by the proud engineering tradition of that great city.
Bit	A bit or binary digit is the smallest unit of data, representing one of two values or states such as 1/0, or yes/no, or love Marmite/hate Marmite.
BJT	A bipolar or junction transistor is a type of transistor with 3 semiconductor layers. See Transistors in the Wiki.
Black Box	Often, a complex system can be broken down into several subsystems. So long as you know the inputs and the outputs of a subsystem and how they're related, you can understand the whole system by regarding that subsystem as a "black box", without considering, or even knowing what's inside it.
BLE	BLE or Bluetooth Low Energy is a variant of Bluetooth offering similar functions to "classic" Bluetooth, but with greatly reduced power requirements. It was originally aimed at applications needing to run for months or years on a single button cell but is now often used in in many regular applications.
Bloatware	Bloatware is the largely useless junk software, often time-limited free trials, bundled by a manufacturer with a Windows computer or an Android tablet or phone. The worst of it is commonly also referred to a crapware (not a term to be used in the most genteel circles). The term may also refer to regular software bloated with functions few uses will need.
Bluetooth	Bluetooth is a personal area networking protocol allowing an individual's personal devices to communicate with one another.
BMS	A BMS or Battery Management System is a circuit designed to control the charging and discharging of a battery, most usually consisting of multiple lithium cells in a battery pack for a laptop, power tool or electric vehicle. Without it, due to the inevitable variation between cells, some cells might be over-charged with potentially dangerous consequences before others have received their full charge, and some might be over-discharged causing permanent damage while others have charge remaining.
BNC	BNC is a type of professional quality coaxial cable twist-to-lock connector, very commonly used for high frequency applications such as in radio, video and testing.
Boost regulator	A boost regulator is a type of switch-mode voltage regulator which produces a higher output voltage than the input.
Bootstrap	A bootstrap is a multi-stage process, for example for starting a computer, in which each stage (starting with a very simple one) enables the next more sophisticated stage to happen, like pulling yourself up by your bootstraps. A bootloader is a program which uses the bootstrap principle to "boot up" a computer into its running state.
Bot	A bot (short for Robot) is a computer which has been compromised in order to autonomously use its Internet connection for spreading spam or malware. Usually it will be one of many such computers comprising a "botnet", under the control of a "bot herder".
Brick	To "brick" a device is to render it completely useless, i.e. having as much functionality as a house brick. This can happen, for example, if the wrong type of firmware is accidentally loaded. "Unbricking" it can then be challenging, sometimes virtually impossible.
Bridge Rectifier	A bridge rectifier consists of four diodes connected in a diamond shape. Applying AC to two opposite corners, you get DC out of the other two. See Linear Power Supplies in the Wiki.
Brush	A brush in an AC/DC motor consists of a small block of graphite, a pair of which feed the current to the rotor via the commutator. See DC and Universal (AC/DC) Motors in the Wiki.
Brushless Motor	An Electronically Commutated or Brushless motor is a type of DC motor which uses an electronic circuit to switch the current between several coils in order to keep the rotor turning, instead of using a commutator and brushes to perform this function. See DC and Universal (AC/DC) Motors in the Wiki.
Brute force attack	A brute force attack is an attempt to hack into an account by trying all possible passwords. It's easy to defend against simply by using a reasonaly strong password.
Buck regulator	A buck regulator is a type of switch-mode voltage regulator which produces a lower output voltage than the input.
Buffer overflow vulnerability	A buffer overflow happens when a program is given more data (whether direct user input or from a network source) than will fit in the memory space that has been allocated for it. The extra may then overwrite nearby data causing unexpected behaviour, which may include an attacker being able to take over control of the program. Buffer overflows can be avoided by strict adherence to a cardinal rule of secure coding: thoroughly validate all input from potentially untrusted sources.
Bug	A bug in software or hardware is a coding error causing unexpected or erroneous operation. The first documented use of the term in respect of computers relates to a dead moth which was causing faulty operation of a relay in an early electro-mechanical computer. However, the term had been used by engineers for many years previously in a wider context.
Bus	A bus in computer terms is a method of allowing serveral modules to communicate. It may consist of any number of parallel wires to which all modules are connected. In some implementations one module will be the "master" and will control all the others, termed "slaves", whilst in others, any module may bid for control of the bus in order to become a temporary master.
Byte	A byte or octet is 8 bits, which can be used to represent a single letter, number or punctuation mark, or a number between 0 and 255, or anything else that can have up to 256 values.
C
Cache	A cache is a small and very fast block of memory used to hold immediate working data or frequently used instructions from a computer program.
Carrier	A carrier is a radio frequency signal which is modulated by an audio signal (for example) so that the audio can be transmitted by radio.
Capacitor	A capacitor or a condenser (an old-fashioned term) is an electronic component which can store electric charge. Its capacitance (or its capacity for storing charge) is measured in Farads. See Capacitors in the Wiki.
Capacitance	Capacitance is the property of storing electric charge measured in Farads, which is the purpose of a capacitor.
Cathode	A cathode is a negative electrode.
CCFL	A CCFL or Cold Cathode Fluorescent Lamp is like a standard fluorescent tube or compact fluorescent lamp except that it doesn't have a heated filament at each end to help start the discharge. Instead, a high voltage is required. They were often used for the backlight in older laptops, TVs and monitors but are now largely displaced by LED strips.
Ceramic resonator	A ceramic resonator is similar in function to a quartz crystal but using a piezoelectric ceramic such as lead zirconium titanate instead of quartz. They are used as a cheaper alternative to a quartz crystal where the precise frequency is not so critical, and also as the tuning filter in the IF stages of some radios.
Choke	An inductor (or choke) is a coil of wire, very often wound around a magnetic core, providing a kind of momentum to the flow of electricity. Its inductance is measured in Henrys. It is commonly known as a choke when used to block high frequencies or reduce ripple from the AC mains. See Inductors in the Wiki.
Chrominance	Chrominance is the hue of a colour. This can be used together with the luminance (the brightness) and the saturation (the purity of the colour) as an alternative to specifying the red, green and blue (RGB) components. This is useful because the eye perceives variations of chrominance and saturation across an image much less sharply than variations of luminance. Hence these latter can be presented at lower definition with no visible degradation in the image, so saving storage or transmission bandwidth. It also allows a graphic designer to select a range of related colours by varying just one of those three parameters.
Circuit	Electricity really hates piling up, so it will only flow if it can go all around a complete circuit and come back to where it started. (The usual symbol for an on/off switch is a circle broken by a dash, representing an interrupted circuit.)
Circuit Diagram	A vacuum cleaner, for example, may contain several paths or "circuits" around which the current can flow, and a computer may contain a huge number. A circuit diagram is a picture of all those paths (or some of them) allowing you to see how the parts are connected together and hopefully understand how it works.
Circular economy	A circular economy is one where a minimum of items or their parts are sent for landfill or incineration, and a high proportion are either reused, repaired or recycled, so minimising the consumption of raw materials and the pre-use carbon footprint.
CISC	CISC or Complex Instruction Set Computer is a computer architecture in which the basic machine instructions are generally fairly complex and highly functional, however, this means that they are relatively expensive to implement. By contrast, the RISC or Reduced Instruction Set Computer architecture was developed to optimise performance in other ways. Intel x86 is the most well known example of this architecture.
CLI	A CLI or Command Line Interface is a type of user interface in which the user types commands at a prompt. Also known as a Character User Interface or CUI (pronounced "coo-ee"). The acronym is also used in telephony to mean Calling Line Identity, i.e. the phone number initiating a call.
Class 1/2/3	In the context a PAT testing, Classes 1, 2 and 3 refer to classifications of equipment according to whether they operate at dangerous voltages and whether they include a protective earth connection. See PAT Testing in the Wiki.
Class A/B/C/D	Class A, B, C and D refer to different design architectures for an amplifier, having different power efficiencies. Class A drives the output throughout a cycle, and is the least efficient. Class B drives the output for only a half cycle. Usually, a second Class B driver drives alternate half cycles. Class C drives the output for less than half a cycle and generally drives a tuned circuit to restore the output (typically in a transmitter) to a pure sine wave. Class D, the most efficient, uses pulse width modulation. The output is switched on and off at a rate well above the audio to create the amplified signal.
Class X/Y	Classes X and Y are designations of capacitors specifically rated for direct connection to the mains supply, bearing in mind the safety implications and the particuar stresses they may be subject to. See Capacitors in the Wiki.
Client	A client in computer terms is a computer (or a program running on a computer) the main purpose of which is to interact with a user. It relies on a server to do the bulk of the processing or data storage.
Cloud	On their network diagrams, computer network engineers frequently represent the Internet or unspecified remote computers by a fluffy cloud. The "cloud" has therefore come to be used to refer to any remote third party computer services or networks. Bear in mind though that the cloud is in reality just someone else's computer. It's worth asking yourself, therefore, how much you trust them with your data!
CMOS	CMOS (pronounced "sea-moss") or Complementary Metal Oxide Semiconductor is a class of very low power logic gates. In early PCs the BIOS settings were held in CMOS memory kept alive by a small backup battery, and the term is still sometimes (erroneously) used for this memory, although Flash memory is now universally used. (The backup battery now only keeps the real time clock alive.)
CMY	A colour image or video can be represented by the red, green and blue (RGB) primary colours added together in appropriate proportions, but this doesn't work for printing, since each colour ink subtracts a range of colours from the incident white light rather than adding anything. So printers use the complementary colours Cyan (green + blue, absorbing red light), Magenta (blue + red, absorbing green light) and Yellow (red + green, absorbing blue light). In practice, printers tend to use black ink as well, and this is referred to as CMYK.
CMYK	Rather than the primary colours red, green and blue, printers use the complementary colours cyan, magenta and yellow (referred to as CMY) but also black to darken a pixel as required. This is then referred to as CMYK.
COB	COB or Chip On Board is a method of construction in which one or more bare semiconductor chips are mounted directly on a circuit board rather than encapsulated in individual packages. It is often used in mass produced items such as calculators in which the chip is covered by a blob of black resin, and more recently to mount many LED chips on a base to form a high power lamp, or on a thin strip to make a filament type of LED lamp.
Collector	The emitter, base and collector are the three terminals of a junction or bipolar transistor. See Transistors in the Wiki.
Collet	A collet is a device for holding a spindle or a rotary tool such as a drill. It consists of a hollow metal cylinder with several slits along part of its length. It is sized so that the tool is a sliding fit inside it. The slits then allow it to be squeezed by some mechanism in order to securely grip the tool.
COM port	A COM port is a serial interface port using a DB-9 connector, offered by virtally all home and personal computers before the advent of USB. Modern computers often still offer simulated COM ports over USB connections for interfacing to simple devices requiring only to be able to exchange a stream of bytes.
Commutator	The commutator is the part of the rotor of an AC/DC motor which receives the current from the brushes. See DC and Universal (AC/DC) Motors in the Wiki.
Compact Fluorescent Lamp	A CFL or Compact Fluorescent Lamp is a type of light bulb consisting of a fluorescent tube bent into a compact shape. More efficient than traditional filament lamps but still falling well short of newer LED lamps. See Types of bulb in the Wiki.
Condenser	A capacitor or a condenser (an old-fashioned term) is an electronic component which can store electric charge. Its capacitance (or its capacity for storing charge) is measured in Farads. See Capacitors in the Wiki.
Condenser microphone	A condenser microphone (typically a high quality or professional one) contains a thin conducting diaphragm exposed to the sound and a fixed metal plate behind it. When charged to a relatively high voltage, the varying capacitance between them generates a varying output. (A condenser is a historical name for a capacitor.)
Conductor	A conductor is a material which conducts electricity. Metals and carbon are conductors.
Conflict Materials	Conflict materials are raw materials largely sourced from conflict regions, where their supply is likely to be used for the purchase of arms or the support of armed groups.
Continuity	Continuity refers to an intentional low resistance electrical path between two points in a circuit. A continuity tester is a device which tests for continuity, often giving an audible beep when continuity is detected.
Cookie	A cookie is a small parcel of data passed back to your browser when you visit a website. When you return to the site or navigate to another page on it, your browser will return the cookie with your request. This enables the website to recognise that it's you coming back and so save you having to login or choose your preferences or select the items in your shopping cart all over again.
Coreless motor	A coreless motor is a type of DC electric motor in which the rotor consists of a self-supporting resin-bonded basket of windings rather than having the windings wound around an iron core. See DC and Universal Motors in the Wiki.
Covert channel attack	A side channel or covert channel attack is one which exploits some side-effect of the processing, transmission or storage of data in order to draw inferences about the data itself. For example, unless carefully designed, variations in the power consumed by a processor while encrypting data can leak information about the data, perhaps ultimately leading to discovery of the encryption key. However, the rate of data leakage is usually very low, and insignificant in all but the most secure environments.
Cover A/B/C/D	The main external covers of a typical laptop are sometimes referred to in service manuals and by vendors of spare parts as Cover A: The back of the laptop screen (outer lid); Cover B: The bezel around the screen; Cover C: The palm rest, keyboard area and touchpad; Cover D: The bottom panel, generally giving access to internal components.
CPU	The CPU or Central Processing Unit is the "brains" of a computer which executes a computer program.
Crapware	Crapware is a somewhat vulgar term (best not used in polite company) referring to the largely useless junk software often bundled by a manufacturer with a Windows computer or an Android tablet or phone. (In some circles you might prefer to use the more genteel term bloatware.)
Credential stuffing	A credential stuffing attack is where an attacker uses a list of compromised user names and passwords to try to hack into different systems in the hope (often justified) that a user has used the same password on different sites. This is why you should always use different passwords on different sites.
Crimp	A crimp is a type of connection where a wire is inserted into a tubular receptacle which is then crushed onto the wire with a special purpose crimping tool. Correctly formed, a crimped connection is generally stronger than a soldered one, and can be done faster and more reliably in a production environment.
CRM	CRM or Critical Raw Materials are those raw materials in critically short supply. This may be because there simply isn't much of them waiting to be mined, or because what there is is largely controlled by unfriendly or potentially hostile nations.
CRT	A CRT or Cathode Ray Tube is the image-producing device in old fashioned televisions, computer monitors, radar sets and oscilloscopes. It consists of an evacuated glass enclosure in which electrons are fired at a fluorescent screen to create the image. Superseded by much less bulky LCD displays.
Critical Raw Materials	CRM or Critical Raw Materials are those raw materials in critically short supply. This may be because there simply isn't much of them waiting to be mined, or because what there is is largely controlled by unfriendly or potentially hostile nations.
Crystal	A crystal is most often taken to mean a quartz crystal, but may sometimes refer to the diode (these days germanium but originally a crystal of galena) in a crystal set.
Crystal set	A crystal set is the simplest form of radio. It needs no batteries or other power source apart from the energy provided by the radio waves it receives. Modern crystal sets use a germanium diode to recover the audio from the radio signal but the earliest (pre-war) examples used a crystal of galena, hence the name. See How Radios Work in the Wiki.
CSM	CSM or Compatibility Support Module is an option which may be present and enabled in a UEFI to support booting from legacy MBR-formatted disks. See UEFI and GPT.
CUI	A Character User Interface or CUI is a user interface operated exclusively with a keyboard by typing commands at a prompt or selecting options from a menu. This is in contrast to a Graphical User Interface. Also known as a Command Line Interface or CLI.
CT	A centre tap (CT) is a connection made at the mid point of a transformer winding. The two halves can then produce or be fed by two AC voltages equal in magnitude but opposite in phase. In a power supply this allows full wave rectification with 2 diodes instead of 4. In a Class B amplifier the two halves may be driven in opposite phase by two power transistors. In a balun the centre tap allows for the creation of equal and opposite phases, giving a degree of immunity to interference in a long cable.
Custom ROM	A custom ROM is an alternative to the operating system provided by the vendor for a device such as a smartphone.
D
D-subminiature	D-subminiature connectors are a series of electrical connectors characterised by their D-shaped shell. Whilst there are numerous variants, just a few are or have been in common use. DB-9 (more correctly DE-9) and DB-25 were ubiqitus on home and personal computers as the serial (COM) and paralel ports before being displaced by USB, and DE-15 is still used for VGA video connectors.
DAB	DAB or Digital Audio Broadcasting is the standard used for digital broadcast radio.
Daughter board	A daughter board is a subsidiary circuit board which plugs into a main circuit board, or motherboard.
dB	Decibels (dB) are a measure of relative power, for example by what factor does an amplifier increase the power of a signal, or by what factor is a TV signal reduced by losses in the cable between the aerial and the receiver. It's a logarithmic scale, 3dB representing a doubling of power, hence the dB gains or losses in a signal path can simply be added. Note also that a doubling of the voltage into a given load also doubles the current, the power is increased four-fold, or by 6dB.
DB-9	D-subminiature connectors are a series of electrical connectors characterised by their D-shaped shell. Whilst there are numerous variants, just a few are or have been in common use. DB-9 (more correctly DE-9) and DB-25 were ubiqitus on home and personal computers as the serial (COM) and paralel ports before being displaced by USB, and DE-15 is still used for VGA video connectors.
DB-25	D-subminiature connectors are a series of electrical connectors characterised by their D-shaped shell. Whilst there are numerous variants, just a few are or have been in common use. DB-9 (more correctly DE-9) and DB-25 were ubiqitus on home and personal computers as the serial (COM) and paralel ports before being displaced by USB, and DE-15 is still used for VGA video connectors.
DC	DC or Direct Current is an electrical supply which goes in one direction only. See AC and DC in the Wiki.
DCE	DTE or Data Terminal Equipment and DCE or Data Communications Equipment are the names given to the two ends of an RS-232 data connection. These were originally assumed to be a computer or computer terminal and a modem respectively but the DCE is now more commonly a computer and the DTE a device or computer controlling it through a command line interface.
DDOS	A DDOS or Distributed Denial of Service attack is one launched simultaneously from many different computers across the Internet, so increasing its power and making it harder to defend against.
DE-15	D-subminiature connectors are a series of electrical connectors characterised by their D-shaped shell. Whilst there are numerous variants, just a few are or have been in common use. DB-9 (more correctly DE-9) and DB-25 were ubiqitus on home and personal computers as the serial (COM) and paralel ports before being displaced by USB, and DE-15 is still used for VGA video connectors.
Debugging	Debugging is the process of finding and eliminating bugs in an item of software or hardware.
Decibel	Decibels (dB) are a measure of relative power, for example by what factor does an amplifier increase the power of a signal, or by what factor is a TV signal reduced by losses in the cable between the aerial and the receiver. It's a logarithmic scale, 3dB representing a doubling of power, hence the dB gains or losses in a signal path can simply be added. Note also that a doubling of the voltage into a given load also doubles the current, the power is increased four-fold, or by 6dB.
Decimal	Decimal is the number system we commonly use in everyday life, based on the digits 0 - 9.
Decoupling capacitor	Variations in the current drawn by one part of a circuit can affect the voltage of the supply, hence affecting another part of the circuit. "Decoupling capacitors" are commonly connected to a supply at various points in order to absorb these variations and minimise any such unwanted and detrimental couplings.
Demodulation	Demodulation is the process of recovering an audio or TV signal (for example) from the RF signal received from an aerial.
Denial of Service	A DOS or Denial of Service attack is an attack which renders a service (such as a website) unusable for its proper purposes. See also DDOS.
Desktop Manager	The Desktop Manager is that part of an operating system which represents the computer's applications and tools on the screen as items on a desk.
DIAC	A DIAC is a 2-terminal electronic component comprising 3 or 4 semiconductor layers which abruptly switches from a non-conducting to a conducting state when a certain voltage is applied, typically in the region of 30V. It then remains conducting until the current falls below a small holding current. Commonly used in conjunction with a triac in motor speed controllers and older dimmer switches.
Dielectric	A dielectric is an insulating material in which the positive and negative charges in the atoms or molecules can be displaced slightly in opposite directions from their natural positions by an electric field, causing the material as a whole to exhibit an electrical charge.
Digital	Digital is a method of representing, processing and transmitting a signal or a quantity as a number or sequence of numbers, usually expressed in binary as ones and zeros.
Digital Certificate	A digital certificate is a piece of data which declares an association between an identity (e.g. the owner of a website or email address, or a software publisher) and a public key. Anyone can then use the public key to verify a digital signature applied to any piece of data by the identity. The certificate is normally itself digitally signed by a chain of more authoritative identities, ending in a well known root certificate which everyone trusts.
Digital Signal Processor	A DSP or Digital Signal Processor is a microprocessor optimised for demanding real-time computing tasks such as audio or video compression, processing or analysis.
DIL	DIL or Dual In-Line is a type of IC packaging with 2 parallel rows of pins, designed for through-hole assembly.
DIMM	A DIMM, or Dual In-line Memory Module (sometimes known as a "RAM stick") is a small circuit board, one or more of which very commonly serve as main working memory in a computer. It has contacts on both sides of an edge which mates with a socket (hence "Dual In-line") and contains a number of individual memory chips. There are several different standard sizes and each may contain various types and speeds of memory chip. An SODIMM is a smaller variant commonly used in laptops.
Diode	A diode is a 2 terminal electronic component allowing a current to flow in one direction but not the other. See Diodes and Rectifiers in the Wiki.
Dipole	A dipole is a thing with two opposite ends, such as a magnet (with North and South poles), a molecule with a positive charge at one end and a negative charge at the other, or a type of aerial consisting of two rods placed end to end.
DisplayPort	DisplayPort is a standard for carrying digital video from a source such as a computer to a display. It can also carry digital audio, USB and other types of data. The connectors are rectangular but with one corner shaved off. There are also mini and micro versions.
Distro	A distro is a flavour of Linux comprising a kernel, a desktop manager and a selection of utilities.
DLP	DLP or Digital Light Processing is a class of data projector using a Digital Micromirror Device to produce the image.
DMD	A DMD or Digital Micromirror Device is an imaging device in which the brightness of each pixel is determined by an individual tiny electrically operated mirror.
DMM	A DMM or Digital Multimeter is a Multimeter with digital readout.
DNS	DNS or the Domain Name System is the Internet-wide distributed system for looking up the IP address of a domain, such as restarters.net, required for routing data. Your computer queries a pre-configured DNS server which in turn queries other DNS servers for IP addresses it doesn't already know.
DOA	DOA or Dead on Arrival refers to a brand new item which fails to work when first unboxed and switched on.
DOS	A DOS or Denial of Service attack is an attack which renders a service (such as a website) unusable for its proper purposes. See also DDOS.
DOS (Microsoft)	MSDOS (Microsoft Disk Operating System, often simply referred to as DOS) was a Microsoft operating system predating Windows, and lacking a graphical user interface.
Downcycling	Downcycling is recycling where the recovered material is of lower quality than that obtained from primary sources and hence unsuitable for more demanding applications. For example, recycled plastic may contain contamination from its previous use and hence cannot be used for food packaging, and steel from scap cars can be used to make steel girders but not high quality engineering steels. Most recycling is in fact downcycling.
DPDT	A DPDT (Double Pole Double Throw) switch is just two SPDT (or two-way) switches ganged together and operated in tandem. It's used to simultaneously switch two separate circuits between alternate paths.
DPST	A DPST (Double Pole Single Throw) switch is just two SPST (simple on/off) switches ganged together and operated in tandem. It's often used to cut both the live and neutral from a mains supply for safety.
Drain	The source, gate and drain are the three terminals of a field-effect transistor (FET). See Transistors in the Wiki.
DSP	A DSP or Digital Signal Processor is a microprocessor optimised for demanding real-time computing tasks such as audio or video compression, processing or analysis.
DTE	DTE or Data Terminal Equipment and DCE or Data Communications Equipment are the names given to the two ends of an RS-232 data connection. These were originally assumed to be a computer or computer terminal and a modem respectively but the DCE is now more commonly a computer and the DTE a device or computer controlling it through a command line interface.
Duplex	A duplex comunication channel is one that supports communication in both directions. A half-duplex channel only allows communication in one direction at a time whereas full duplex allows for simultaneous communication in both directions.
DUT	A DUT (Device Under Test) is a term used in testing and measurement to refer to the specific electronic component, assembly, or system that is being evaluated.
DVI	DVI is a standard for carrying analogue and/or digital video from a source (most commonly a computer) to a display device. It uses a connector which is almost rectangular, but with two corners on the same long edge slightly rounded. The connectors can accommodate 3 rows of 8 pins plus an extra 4 in a separate group for analogue video, but the number actually present varies according to the type and number of video channels supported.
Dynamic microphone	A dynamic microphone contains a permanent magnet and a coil which vibrates in sympathy with the sound, inducing a small voltage in the coil by the principle of magnetic induction.
E
Earth	Earth is the 3rd planet from the Sun. Being common to all earthlings, in an electronical context earth or ground is taken as the zero of all voltages. A metal stake driven into the ground may be used to obtain a physical earth or ground connection. (The difficulty of providing an earth connection is one of the challenges that will face any furure colonisers of Mars.)
Earth Loop	When two pieces of audio equipment are connected together, each having its own earth connection, some of the small current that should be going from one of the pieces of equipment to its own earth connection instead goes via the connection to the other equipment's earth, causing an objectionable hum in the audio output. This is known as a Ground or Earth Loop.
E-Ink	E-Ink is a type of display, usually monochrome only and commonly used in e-book readers. It has the advantage of using very little power, and can be viewed by reflected ambient light rather than needing an active backlight.
E-Series	The E series (E3, E6, E12, E24 etc) is a system of standard values for resistors, capacitors, inductors and zener diodes dividing each decade equally on a logarithmic scale. So for example, E12 resistors come in 12 values per decade, each roughly 20% greater than the last. See E Series in Wikipedia for more details.
e-Waste	e-Waste is all those electrical and electronic goods discarded by their owner, whether because they are faulty, no longer needed, or simply replaced by a newer model.
ECC	ECC or Error Correcting Codes are additional bits added to a block of data, derived mathematically from the data and enabling errors arising during storage or transmission (up to a point) to be detected and corrected.
Ecodesign	Ecodesign is the integration of environmental aspects into the product development process, by balancing ecological and economic requirements.
EHT	EHT or Extremely High Tension is the name given to the very high voltage supply (usually in excess of 1,000V) required by cathode ray tubes in old fashioned televisions, magnetrons in microwave ovens and a few related devices. Needless to say, it can be very dangerous!
Electret	An electret is a material having a permanent built-in electric charge, like the electrical equivalent of a permanent magnet.
Electret microphone	An electret microphone is one containing an electret material (having a built-in electric charge) to convert the sound into a varying electric charge. An FET is always incorporated in it requiring a supply of a few volts, to convert the charge into a varying voltage.
Electricity	Electricity: tamed magic... well, mostly tamed.
Electrode	An electrode is an electrical conductor making contact with a non-metallic part of a device such as a semiconductor or the electrolyte in a battery or the vacuum in a valve.
Electrolyte	An electrolyte is a conducting liquid containing negative and positive ions, such as the liquid in a battery.
Electrolytic Capacitor	An electrolytic capacitor is a type of capacitor which employs an electro-chemical process to achieve a high value of capacitance. However, their failure rate is relatively high - see Capacitors in the Wiki.
Electromagnet	An electromagnet is a coil of wire wound around an iron core. A current passing through the coil magnetises the iron for only as long as the current flows. Typically used in electro-mechanical devices such as toasters, where it holds the lever down while the toast is toasting.
Electromagnetic Wave	An electromagnetic or EM wave is a kind of wave in space in which energy is continually exchanged between electric and magnetic fields similarly to how energy in a ripple on a pond is continually exchanged between gravity and momentum. Radio waves, light, x-rays and gamma rays are all EM waves of different wavelengths.
Electron	An electron is a sub-atomic particle carrying a negative electrical charge. A flow of electrons constitutes an electric current.
Electronically Commutated Motor	An Electronically Commutated or Brushless motor is a type of DC motor which uses an electronic circuit to switch the current between several coils in order to keep the rotor turning, instead of using a commutator and brushes to perform this function. See DC and Universal (AC/DC) Motors in the Wiki.
Electronicals	Electronicals is a portmanteau word meaning both simple electrical items and electronic devices of any complexity. The term was coined accidentally through a slip of the tongue by @Danielle and quickly adopted by the rest of the team who were fed up with the cumbersome "electricals and electronics".
Electrophoresis	Electrophoresis is the motion of particles through a liquid in which they are dispersed, attracted by an electric field. It's the principle on which most e-ink displays are based and is also extensively used in the analysis of chemical and biological samples, including DNA sequencing.
EMF	EMF or Electro-Motive Force is just a fancy name for Voltage, most often used in the context of a dynamo, or the dynamo effect in an electric motor.
Emitter	The emitter, base and collector are the three terminals of a junction or bipolar transistor. See Transistors in the Wiki.
Encryption	Encryption is the process of transforming a message or data (the "cleartext") into a completely unintelligible form (the "ciphertext"). Only with knowledge of a secret key can the ciphertext be transformed back into the cleartext.
Endurance	Unlike a hard disk, the memory cells in a solid state disk (SSD) are degraded slightly every time they are written to. The Endurance of an SSD is the total amount of data that you can expect to be able to write to it in its useful lifetime. This will be many times its actual capacity.
Ensemble	In the context of DAB radio, an ensemble is the result of multiplexing together the digital audio streams from a number of different stations into a single transmitted digital data stream. See How radios work in the Wiki.
Entropy	Entropy (in information theory) is the amount of information or uncertainty in a piece of data, usually measured in bits. For example a 3 digit PIN or combination lock has 1,000 possible values and so has almost 10 bits of entropy, because 10 bits allows you to count up to 1,024. The entropy of a password or encryption key can be used directly to estimate how long it might take to crack, by trying all possible values.
EoP	An Escalation of Privilege (or EoP or PrivEsc) vulnerability is a software flaw which allows an attacker, already operating on your computer, to increase the privileges allowed to him so as to gain full control.
Epoxy	Epoxy is a type of very strong adhesive which comes as a resin and a separate hardener. It starts to set only when the two are mixed together.
ES	ES or Edison Screw is a screw-fitting lamp base. Several sizes are in common use. See Types of bulb in the Wiki.
Escalation of Privilege	An Escalation of Privilege (or EoP or PrivEsc) vulnerability is a software flaw which allows an attacker, already operating on your computer, to increase the privileges allowed to him so as to gain full control.
ESD	ESD or Electrostatic Discharge is static caused by friction such as shoes on a carpet, and particularly noticeable in conditions of low humidity. Even if not perceptible, it can damage sensitive electronic components.
ESP	ESP or EFI System Partition is a partition on a GPT-formatted disk containing system and boot information. See UEFI and GPT in the Wiki.
Ethernet	Ethernet is a family of networking protocols defining several methods by which packets of data can be transmitted across a computer network.
EV	An EV or Electric Vehicle is any kind of vehicle more conventionally powered by fossil fuels (or pedal power), but designed instead to be electrically powered.
F
Farad	The Farad is the unit of capacitance. Named after British scientist and experimenter Michael Faraday, who made important discoveries in the fields of electromagnetism and electrochemistry. See Capacitors in the Wiki.
FAT	FAT or File Allocation Table is a type of filing system invented by Microsoft for use on DOS and earlier versions of Windows. Its several variants (FAT16, FAT32, ExFat) are supported by a very wide range of operating systems. It has the advantage of simplicity but is limited in the size of disks it can support and doesn't support file attributes such as file permissions.
Feature phone	A feature phone is generally taken to mean a mobile phone offering features beyond simply making and receiving voice calls. These might include a calendar, voice notes or a few simple games, all through a character-based non-graphical screen.
Ferrite	Ferrite is a magnetic ceramic material composed largely of iron oxide. Since it doesn't conduct electricity it's useful for coils and inductors required to operate at high frequencies, since wasteful circulating currents would be induced in iron if this were used.
Ferrite rod aerial	A ferrite rod aerial is a type of aerial used for medium and long wave radio reception and consisting of a rod or block of ferrite (a magnetic but insulating material) with one or more coils of wire wound around it. See How radios work in the wiki.
FET	A FET or field-effect transistor is a type of transistor where the current flowing between two of its terminals (the source and the drain) is controlled by an electric field created by a voltage on a third terminal (the gate). See Transistors in the Wiki.
FFC	An FFC or Flat Flexible Cable is a type of cable with parallel conducting tracks on a plastic ribbon. Used extensively for connections between separate modules in many types of device.
Fibre Optics	If you feed light into the end of a thin glass or transparent plastic fibre, it will be constrained to the fibre by total internal reflection and emerge at the far end, even if the fibre is many miles long. Furthermore, with high quality fibre the losses can be much less than with an electrical cable, and light can carry vastly more data than an electrical signal. Fibre optics is the application of this principle, together with the ancillary lasers or LEDs for generating the light and the detectors for receiving it at the far end.
Filament Lamp	A filament lamp is a type of light bulb containing a thin tungsten wire which glows white hot when a current is passed through it. Such bulbs are highly inefficient and can no longer be legally sold. See Types of bulb in the Wiki.
Filing System	The filing system is that part of an operating system which manages a disk and presents it to the user as individual named files, hiding all the messy details about where on disk the data is actually stored.
Firewall	A firewall is a networking device which controls the traffic passing through it according to a defined policy, typically to prevent hackers from accessing a private network, or to block maliciously crafted requests.
Firewire	Firewire (also known as IEEE 1394) is a high speed data interface mainly used for video and external hard disks.
Firmware	Firmware is the Software which is built in to the hardware of a computer or smart device and is expected to be rarely if ever changed.
Flash	Flash is a term used for a type of memory that can only be changed by erasing and rewriting a substantial block of cells at a time. Flashing is used as a verb for the process of rewriting or updating such a memory.
Floating Point	Many of the numbers a computer deals with are whole numbers (integers) which can be easiy represented in binary, but real world numbers can span a huge range of sizes. Floating point is a method of representing such numbers in two parts: a mantissa represents the significant digits and an exponent tells you where the decimal point comes (or the binary point for binary numbers). This is similar to the scientific notation in which one and a half million would be written as 1.5x106.
Flux	Flux is a liquid or paste which helps to remove oxide or other deposits in order to make good solder joints. Only usually needed for challenging soldering jobs such as surface mount as solder normally contains a core of flux.
FM	FM or Frequency Modulation is a method of modulation in which the frequency of an RF signal is modulated by the AF signal to be transmitted. See Frequency Modulation in the Wiki.
FPC	An FPC or Flexible Printed Circuit. A flexible substrate with printed circuit tracks, generally brown or dark orange, and often having surface mount components or connectors soldered directly onto it.
FPGA	A Field-Programmable Gate Array is an integrated circuit containing a large number of uncommitted simple circuit elements such as logic gates. All the interconnections can be programmed after manufacture to create a complex subsystem of the designer's choosing.
Frequency	Frequency is the number of complete (positive and negative) cycles of AC which occur per second. Measured in Hertz.
FTP	FTP (or File Transfer Protocol) is an insecure method of transferring files from one computer to another, such as from your computer to your own website hosting provider. Secure alternatives SFTP or FTPS are strongly recommended instead.
FTTC	FTTC or Fibre To The Cabinet (Part-Fibre) is a broadband Internet connection delivered through optical fibre only to a street cabinet, and copper wires for the remainder to the customer premises, as opposed to the faster FTTP which is fibre all the way.
FTTP	FTTP or Fibre To The Premises (Full Fibre) is a broadband Internet connection delivered through optical fibre to the customer premises, as opposed to the slower FTTC which is fibre only to a street cabinet, and copper wires for the remainder.
FUBAR	Fouled Up Beyond All Recognition is an expression used to describe a situation which is about as bad as it could be. If anything it's rather worse than a SNAFU. (There are other interpretations of the acronymn conveying the same or a similar meaning but not all suitable for polite company.)
Full wave rectifier	A full wave rectifier is one that which uses both the positive and negative half cycles of the AC input to produce a DC output. Most often this is achieved with a bridge rectifier.
Fuse	A fuse is a thin piece of low melting point wire designed to melt and so break the circuit if too much current flows due to a fault.
G
Gallium	Gallium is a soft metal similar to aluminium but with a low melting point. In electronics, many compounds of gallium are semiconductors with useful properties. Gallium arsenide devices are able to operate at much higher frequencies than their silicon counterparts and so are widely used in mobile phones and satellite comunications. Various other gallium compounds are used in practically all visible and infra-red LEDs.
Garbage collection	Garbage collection is the process of collecting up disused memory or storage to allow it to be reused. In an SSD it refers to the internal process by which it consolidates still needed data scattered amongst disused data in order to be able to erase complete data blocks for reuse.
Gate	The source, gate and drain are the three terminals of a field-effect transistor (FET). See Transistors in the Wiki.
GB	A GB or Gigabyte is 1000 megabytes, but the term is often used for 1024 megabytes, more properly written GiB.
Germanium	Germanium is a semiconductor used for the manufacture of early transistors. It has been superseded by silicon for nearly all purposes but is still used in conjunction with other materials in some applications such as solar cells and LEDs.
GHz	A GHz is a billion (thousand million) Hz
GiB	A GiB is 1,024MiB, or 1,073,741,825 bytes. It is as much as can be addressed with 30 bits.
Gnat’s whisker	A gnat’s whisker a measure of distance used by engineers, being rather less than you can see with the naked eye and definitely less than a smidgeon. Despite much discussion in corridors and around the water cooler, it seems the British Standads Institute has never been able to decide on a standard gnat, there being so many diptera in the suborder Nematocera.
GNSS	GNSS or Global Navigation Satellite System is an umbrella term for the several satellite-based Position, Navigation and Timing (PNT) systems. The core systems are GPS (United States), GLONASS (Russian Federation), Galileo (EU) and BeiDou (China). They all rely on the fact that you can determine your latitude, longitude, altitude and the time by comparing the relative delays of highly accurate timing signals from at least 4 satellites within direct line of sight.
GPS	GPS or Global Positioning System is a satellite navigation system allowing receivers to determine their geographical position by comparing the delays in time of arrival of signals from a minimum of 4 satellites. The US, Russia, the EU and other countries each have their own systems but a suitable GPS receiver can combine the results from more than one system to improve its accuracy.
GPT	GPT or GUID Partition Table is the name given to a newer method of organising the contents of a disk. See MBR vs GPT disk layout in the wiki.
Grid	The grid is the control electrode in a valve. Driving it negative with respect to the cathode restricts or prevents a flow of electrons from the cathode to the anode.
Ground	Earth is the 3rd planet from the Sun. Being common to all earthlings, in an electronical context earth or ground is taken as the zero of all voltages. A metal stake driven into the ground may be used to obtain a physical earth or ground connection. (The difficulty of providing an earth connection is one of the challenges that will face any furure colonisers of Mars.)
Ground Loop	When two pieces of audio equipment are connected together, each having its own earth connection, some of the small current that should be going from one of the pieces of equipment to its own earth connection instead goes via the connection to the other equipment's earth, causing an objectionable hum in the audio output. This is known as a Ground or Earth Loop.
GUI	A GUI (pronounced "goo-ey") or Graphical User Interface is a user interface operated principally using a graphical display and a mouse or other pointing device, as opposed to a Character User Interface, operated just by a keyboard and text on a display.
GUID	A GUID or Globally Unique Identifier is a 128 bit number used for identifying various types of resource in a computer system. A 128 bit number is so large that if you choose one at random, there is practically no chance that anyone else will ever pick the same one, even if you were to wait billions of years.
H
Half wave rectifier	A half wave rectifier is one which uses only the positive (or the negative) half cycles of the AC input to produce a DC output, by using a single diode.
Hall Effect	When a current flows through a conductor and a magnetic field is applied to the conductor at right angles, the current tends to be pushed to one side of the conductor, at right angles to the magnetic field. This is the Hall Effect. Used in a Hall effect device to detect or measure a magnetic field, for example in smartphones to detect the earth's magnetic field to implement a compass.
Halogen Lamp	A halogen lamp is an improved form of filament lamp with longer life and better efficiency, but still falling far short of compact fluorescent and LED lamps. See Types of bulb in the Wiki.
Haptics	Haptics are technologies involving the sense of touch, such as the vibrate alert option offered by most smart phones.
Hard Disk	A hard disk or HD is a common type of computer main long term storage in which data is held on a rapidly spinning magnetic disk. (Increasingly, modern computers use a Solid State Disk instead.)
Hardware	Hardware is the physical electronic and electromechanical parts which comprise a computer.
HD	A hard disk or HD is a common type of computer main long term storage in which data is held on a rapidly spinning magnetic disk. (Increasingly, modern computers use a Solid State Disk instead.)
HDMI	HDMI is a standard for carrying digital video and audio from a source such as a computer or a TV tuner to a display device such as a flat screen monitor or a projector. It uses a connector in standard, mini and micro sizes, all shaped as a rectangle with two chamfered corners on the same long edge.
Heat Sink	A heat sink is a piece of metal designed to conduct heat away from a component so as to prevent it getting too hot. When soldering, a pair of pliers can be applied to a lead of a sensitive electronic component as a heat sink to prevent the heat of the solder from damaging the component.
Henry	The Henry is the unit of inductance. Named after American scientist Joseph Henry, who made important discoveries in the field of electromagnetism (independently of Michael Faraday). See Inductors in the Wiki.
Hertz	The Hertz (Hz) is the unit of frequency, numerically equal to cycles per second. Named after German physicist Heinrich Hertz, who was the first to prove the existence of previously theorised electromagnetic waves.
Hexadecimal	Hexadecimal is a number system based on 16 digits which are represented by 0-9 and A - F. This is useful because each hexadecimal digit can represent 4 bits, and 2 hexadecimal digits can represent 8 bits or a byte.
Hole	A hole is the lack of an electron in a crystal lattice where there should be one. This leaves behind a positive charge. An electron from an adjacent atom can move into the hole causing the hole to move to the next atom. In this way, holes can act like positively charged particles and carry an electric current.
Horology	Horology is the theory and practice of timekeeping and clock-making.
HT	HT or High Tension is the name given to the main high voltage supply required by valve equipment, usually in the region of several hundred volts. Warning: this can be dangerous!
http/https	http or Hypertext Transfer Protocol is the method by which a web browser requests and receives web pages from a web server. It is now largely superseded by https, which transmits requests and replies in encrypted form.
Hum	Hum is a constant low frequency sound in the output of audio equipment due (for example) to inadequate smoothing in the power supply and residual ripple in its output, or to the presence of a ground loop. Alternatively, it's what you do when you don't know the words.
Hypervisor	A hypervisor is a program which allows you to run one or more virtual machines, each in its own padded cell and hence prevented from interfering with one another or with the hypervisor itself. A hypervisor may run as an application under a host operating system, or may run on the "bare metal" without an operating system, and hence free to utilise the totality of available computing, memory and peripheral resources.
Hz	The Hertz (Hz) is the unit of frequency, numerically equal to cycles per second. Named after German physicist Heinrich Hertz, who was the first to prove the existence of previously theorised electromagnetic waves.
I
I2C	I2C (Inter-Integrated Circuit, also written I²C or IIC) is a simple protocol allowing a master device to exchange low to medium speed data with a number of slave devices, all connected to a common bus. Requiring just two wires for a serial clock (SCL) and serial data (SDA) it's simple and cheap to implement. Although rarely seen, it is ubiquitous, being used in a computer to communicate presence, type, capabilities and status information with monitors, memory modules, batteries etc., as well as in many other applications. Raspberry Pi and Arduino, popular with hobbyists and the maker community, use it for communicating with a whole host of peripheral devices.
I2S	I2S (Inter-IC Sound, also written I²S or IIS) is a simple protocol for the transmission of uncompressed serial digital audio. It is intended for communication between modules within a single device, and hence no inter-device connecting cables are defined.
IC	An Integrated Circuit or IC is a circuit comprising anything from dozens to billions of electronic components, all fabricated on a single silicon chip. See Integrated Circuits in the Wiki.
IF	IF or Intermediate Frequency is a frequency between the RF and AF in a superhet radio at which most of the amplification and selection of the required station occurs.
IGBT	An Insulated Gate Bipolar Transistor or IGBT is a device in which a MOSFET and a bipolar transistor are integrated into a single structure offering many of the advantages of both types of device. IGBTs are therefore capable of operating at both very high voltages and very high currents.
IMAP	If you manage your email using a program or app on your own device (rather than using webmail) then IMAP or Internet Message Access Protocol provides a method by which your program or app can manage your email, optionally organised into different folders, held on a remote email server, and to retrieve emails as required.
Impedance	For most practical purposes, impedance is the same as resistance, and also measured in ohms. Resistance is the number of steady volts you need to push a steady amp through a circuit, but if the voltage is AC or is varying, then inductance and capacitance add a kind of momentum and springiness to the way the current responds to the voltage. The volts per amp is then likely to be different, and is termed impedance.
Incandescent Lamp	A filament lamp is a type of light bulb containing a thin tungsten wire which glows white hot when a current is passed through it. Such bulbs are highly inefficient and can no longer be legally sold. See Types of bulb in the Wiki.
Inductance	Inductance is the property of a coil of wire (or even a straight piece of wire) of storing energy in the form of a magnetic field generated by a current flowing through it. It is measured in Henrys. When the voltage driving it is reduced or withdrawn, this energy is released back into the current, giving it a kind of momentum and tending to keep it flowing.
Induction motor	An induction motor is a type of electric motor which runs only on AC. Often used in fans. See Induction Motors in the Wiki.
Inductor	An inductor (or choke) is a coil of wire, very often wound around a magnetic core, providing a kind of momentum to the flow of electricity. Its inductance is measured in Henrys. It is commonly known as a choke when used to block high frequencies or reduce ripple from the AC mains. See Inductors in the Wiki.
Inrush	Inrush is a momentary surge of current drawn by a device on first switching it on, often causing a noticeable dimming of lights for a fraction of a second. Some equipment is fitted with a slow blow fuse to withstand the inrush. Items containing a large electric motor often have a significant inrush current as the motor starts.
Instruction set	The instruction set of a computer is the repertoire of fundamental low-level machine istructions used to program it. Different computer architectures such as x86, ARM and RISC-V have different incompatible instruction sets.
Insulation displacement	Insulation displacement is a type of connection where insulated wire, without stripping the insulation, is forced into a slot with sharp inner edges. These edges cut through the insulation and bite into the wire. Commonly used on telephone and network connectors, and connectors designed for attachment to ribbon cables.
Insulator	An insulator is a material which doesn't conduct electricity. Examples are plastic, wood, glass and fresh air.
Integrated Circuit	An Integrated Circuit or IC is a circuit comprising anything from dozens to billions of electronic components, all fabricated on a single silicon chip. See Integrated Circuits in the Wiki.
Interface	An interface in computer terms is a connection between two (or more) computers or components of a single computer, or the means by which a computer interacts with the human users. In the former case it will be defined by the types of plug and socket used, and a specification of the signals passed across and their meanings.
International Octal	IO or International Octal was a type of 8-pin valve base with a keyed spigot, used on older and larger valves, and often referred to simply as "octal".
Internet	The Internet is a global computer network in which (unlike the telephone network) information is transmitted as relatively small packets of data, each packet individually addressed and individually routed to its destination. Many different services use this foundation, two of the best known being the World Wide Web (which delivers web pages) and email.
Internet of Things	The Internet of Things (or IoT) refers to the networking of smart devices which have not traditionally been thought of as computers, such as vehicles, security cameras, home heating systems, kitchen appliances and medical devices, as well as industrial machines and sensors. As a result, they can be remotely monitored and controlled or can feed data back to a service provider. However, if they are not properly secured (as is often the case) they can represent a serious security risk.
Intranet	An intranet is a computer network using the same technology as the Internet but existing only within the confines of a particular company or organisation. However, most intranets have a gateway through which users can access the wider Internet, usually subject to certain restrictions.
Ion	An ion is a positively or negatively charged atom.
IoT	The Internet of Things (or IoT) refers to the networking of smart devices which have not traditionally been thought of as computers, such as vehicles, security cameras, home heating systems, kitchen appliances and medical devices, as well as industrial machines and sensors. As a result, they can be remotely monitored and controlled or can feed data back to a service provider. However, if they are not properly secured (as is often the case) they can represent a serious security risk.
IP	IP or Internet Protocol is a set of standards defining how data (whether it be it web pages, emails or anything else), broken up into individually addressed packets, is moved across the Internet from a source to a destination. There are two versions in use: predominantly Version 4 (IPv4), and also version 6 (IPv6), which allows vastly more addresses.
IPAddress	An IPAddress is the equivalent of a telephone number on the Internet, and is used for routing data from one device to another anywhere in the world. IP version 4 addresses consist of 4 decimal numbers separated by dots (e.g. 192.168.2.1), whereas in IP version 6 they consist of up to 8 hexadecimal numbers separated by colons. One or more consecutive zero fields are represented by a double colon, e.g. 2a00:1450:400e:810::200e
IPv4	IP version 4 (IPv4) is the version of the Internet Protocol predominantly in use on the Internet today. It uses 32 bit addresses which has proved insufficient as the reach of the Internet has become universal.
IPv6	IP version 6 (IPv6) is a later version of the Internet Protocol using 128 bit addresses and designed to overcome the limited addressing capabilities of the older IPv4. Although supported by virtually all current networked devices, its adoption has been slow as interworking with IPv4 is far from transparent.
IrDA	IrDA is a suite of communication protocols for data transfer over a short range line-of-sight infrared link. It was commonly used for syncing a feature phone with a laptop but has largely been displaced by Bluetooth in all but a few use cases.
ISO	ISO is the International Organisation for Standardisation which publishes thousands of standards, but in computing the term is often used to refer to one particular standard for the layout of information on a CD or DVD. A Linux or Windows installation file, for example, will often be available as an ISO file, which is a byte-for-byte disk image which can be burned directly to a DVD.
IT	Information Technology, normally used to refer to computers and all ancillary equipment.
J
Jack	A jack is the type of plug (and its socket) familiar as a headphone plug.
jFET	A Junction Field Effect Transistor is a type of field effect transistor where the gate is formed by a semiconductor junction. See Transistors in the Wiki.
JIS	JIS or Japanese Industry Standard is a type of crosshead screw and screwdriver commonest in the Pacific rim. It is distinguished by a raised or recessed dot on the screw head. See Types of screw head and screwdriver in the Wiki.
Joule	The joule is the unit of energy, being the amount of work done by 1 watt in 1 second. 4.2 Joules are equivalent to 1 calorie of heat. The unit is named after English physicist James Prescott Joule, best known for discovering the law which states how much heat is produced in a conductor by the passage of an electric current.
Junction Transistor	A bipolar or junction transistor is a type of transistor with 3 semiconductor layers. See Transistors in the Wiki.
K
Kapton	Kapton, often encountered in the form of adhesive tape, is a material used for insulation and protecting sensitive components on account of its ability to withstand a very wide range of temperatures, including those used in soldering.
kB	A Kilobyte (kB) is 1024 bytes.
Kernel	The core of an operating system, which concerns itself with managing the hardware resources at the lowest level. It normally has no user interface, this being provided by another program or programs referred to as the shell. Think of a nut. The shell is what you see but the goodness is inside in the kernel.
Keylogger	A keylogger is a type of malware or a hardware device which logs your keystrokes (including passwords, credit card numbers etc.) and sends them back to a malicious party, or stores them for later collection.
kHz	A kHz is 1,000 Hz
Kirchoff's Laws	You can calculate the voltages and currents in a simple circuit with Ohm's Law, but not for more complex circuits (such as a decorative net curtain of LEDs). Kirchoff's Laws allow you to write down a series of simultaneous equations which you can solve to find all the voltages and currents. Briefly, (a) the currents flowing into and out of any node in a circuit must be equal, and (b) considering any closed loop in a circuit, the positive and negative voltages around the loop and back to the starting point must sum to zero.
L
Ladder logic	Ladder logic is a kind of simple graphical programming language with its roots in relay logic, formerly much used for the control of industrial plant and machinery, but still used by PLCs in similar applications. See Ladder logic in Wikipedia for more details.
LAN	A Local Area Network is a computer network covering a limited area such as a single building, or at most, a campus.
LCD	A Liquid Crystal Display is the type of display used in most calculators, phones, tablets and computer monitors. It uses a special liquid which acts on polarised light under an electric field.
LDO	A LDO or Low Drop-out regulator is a voltage regulator which produces the required output voltage even when the input voltage is only a little higher.
LDR	A Light Dependant Resistor is a device having a high resistance in the dark but whose resistance falls with increasing illumination.
LED	A LED is a Light Emitting Diode, similar to any other semiconductor diode but specially crafted so that photons are emitted as electrons fall into holes at the junction between a p-type and an n-type region.
LIDAR	LIDAR or Light Imaging, Detection and Ranging is a technology similar to radar but using visible or infrared light instead of radio. As with radar, the distance of an object is determined by measuring the time taken for a pulse of light to be reflected off it and to return.
Lightning (connector)	Lightning is an Apple-proprietary connector used on Apple iPods, iPhones and iPads since 2012. It can be inserted either way up and can be used for charging, sync and accessories. It contains an authentication chip making it difficult for 3rd parties to produce compatible accessories.
Linear Regulator	A linear regulator is a circuit often in the form of an IC which produces a constant well defined voltage from a variable or poorly defined higher voltage input, simply by absorbing excess voltage and turning it into heat. See Linear Power Supplies in the Wiki.
Linux	Linux is a free, open source operating system kernel which, packaged into a distro, can be used as an alternative to Windows or OS/X.
Live Linux	A live linux is a Linux system on a bootable CD/DVD or memory stick which can be run from that media without affecting the installed operating system on the hard disk.
Li-ion	Lithium-ion and lithium-polymer are types of high capacity rechargeable battery much used in portable electronic devices and electric vehicles. See Batteries in the Wiki.
Li-polymer	Lithium-ion and lithium-polymer are types of high capacity rechargeable battery much used in portable electronic devices and electric vehicles. See Batteries in the Wiki.
Load cell	A load cell is a device for measuring a force and presenting it as an electrical signal. See Digital weighing scales in the wiki.
Loupe	A loupe is a magnifying glass, generally without a handle. Jewellers and watchmakers commonly use an eye loupe, which with a little practice can be held in the eye socket, leaving both hands free.
LT	LT or Low Tension is the name given to the low voltage supply required by valve equipment to heat the cathodes of the valves. A common value is 6.3V.
Luminance	Luminance is the brightness of a colour. This can be used together with the chrominance (the hue) and the saturation (the purity of the colour) as an alternative to specifying the red, green and blue (RGB) components. This is useful because the eye perceives variations of chrominance and saturation across an image much less sharply than variations of luminance. Hence these latter can be presented at lower definition with no visible degradation in the image, so saving storage or transmission bandwidth. It also allows a graphic designer to select a range of related colours by varying just one of those three parameters.
M
M.2	M.2 is a type of expansion card commonly (but not exclusively) used for SSDs, especially in ultra-thin laptops where a conventional 2.5" SSD would be too bulky. There are several types distinguished by the position of one or more keyways in the edge connector. See All about SSDs in the Wiki.
Magnetic induction	Magnetic induction is the effect discovered by Michael Faraday, by which a voltage is induced in a conductor subjected to a varying magnetic field. This is the principal on which all dynamos and generators rely.
Magnetic Reluctance	When a magnetic material such as iron is subject to a magnetic field, the atoms tend to line up with the field, increasing its effect. Magnetic reluctance is a measure of how much magnetic field is required to produce a given magnetic effect. Fresh air is non-magnetic and so has a high reluctance, unlike iron, which is very susceptible to magnetism.
Magnetron	A magnetron is the device which produces the microwaves in a microwave oven, originally developed during WWII as a small and high powered source of microwaves for radar applications. A strong magnet causes a stream of electrons to circle around a series of cavities causing them to resonate electrically in much the same way as blowing across the top of a bottle can make it resonate.
Malware	Malware is a term used for any bad piece of software which attempts to get onto your computer for malicious purposes.
MB	A MB or megabyte is 1000 kilobytes, but the term is often used for 1024 kilobytes, which is more properly written MiB.
MBR	The MBR or Master Boot Record is the name given to an older method of organising the contents of a disk. See MBR vs GPT disk layout in the wiki.
MCU	A Microcontroller or MCU (MicroController Unit, sometimes written μC) is an integrated circuit comprising a complete computer on a single silicon chip, albeit generally a relatively simple and low power one. It will contain a processor for executing instructions, program memory, working memory and a range of peripheral devices such as analogue and digital inputs and outputs, timers, counters and communication channels. Microcontrollers are all around us, controlling everything from toasters and washing machines to the door locks and anti-lock brakes in a car as well as all manner of industrial plant and machinery.
Microprocessor	A Microprocessor or MPU (MicroProcessor Unit) is the compete central processor of a computer on a single silicon chip, or in the case of a high-end microprocessor, possibly more than one chip in a single package. Additional components are required to provide program and data storage and peripheral functions such as input and output.
Memory effect	The memory effect is the property of some types or rechargeable batteries to "remember" if they have frequently been only partially discharged. They can then become reluctant to deliver more charge in future. The cure is to fully dischage then fully recharge them periodically. NiCd batteries are particularly prone, and NiMH rather less so.
Memory leak	Sophisticated software such as a browser performs much of its work using small chunks of memory for temporary storage, borrowed from a pool. Sometimes, due to a bug, it may fail to return it to the pool when it's finished with it. This is known as a memory leak, and can result in the memory pool becoming depleted to the extent that the software can no longer operate. The only solution is to close and relaunch it.
MEMS	MEMS or MicroElectroMechanical Systems are devices containing microscopic moving parts and fabricated using the techniques developed by the microelectronics industry for manufacturing integrated circuits. They find many common applications, including the accelerometers and microphones in smartphones, pressure sensors, the print head in some inkjet printers and the Digtal Micromirror Device which creates the image in some data projectors.
Metadata	Metadata is "data about data". For example, a document or spreadsheet may contain information about the author; a digital camera will normally embed information about the exposure, location, date and time of a photo. Sharing these or other types of file can result in undesirable information leakage and privacy issues if you're not aware of the metadata they may contain.
Metal Halide Lamp	A metal halide lamp is a very efficient type of lamp which operates by creating an electrical discharge through a mixture of gasses such as mercury vapour and a metal iodide or bromide (the "halide") in order to give the full spectrum of white light. They are not used domestically except as the light source in a data projector as they can take several minutes to reach full brightness, run very hot, and can explode if abused. They are frequently used for commercial lighting (e.g. in shops) and for floodlighting.
MFA	Two Factor Authentication (2FA) and Multi Factor Authentication (MFA) are authentication schemes requiring a user to pass two or more tests, different in kind, in order to prove their identity. These are usually chosen from something you know (for example a password), something you have (a physical token) and something you are (a biometric such as a fingerprint or facial scan). Such a sytem is much harder to compromise than a simple password login.
MHz	A MHz is a million Hz
MiB	A MiB is 1,048,576 bytes, the amount that can be addressed with 20 bits.
Microcontroller	A Microcontroller or MCU (MicroController Unit, sometimes written μC) is an integrated circuit comprising a complete computer on a single silicon chip, albeit generally a relatively simple and low power one. It will contain a processor for executing instructions, program memory, working memory and a range of peripheral devices such as analogue and digital inputs and outputs, timers, counters and communication channels. Microcontrollers are all around us, controlling everything from toasters and washing machines to the door locks and anti-lock brakes in a car as well as all manner of industrial plant and machinery.
MicroSATA	MicroSATA or μSATA is a smaller version of the standard SATA connector used on modern 2.5" and 3.5" disk drives, and used on smaller 1.8" drives.
Microswitch	A microswitch is a type of switch often found in electromechanical devices, typically actuated by a safety interlock, a cam or a lever rather than directly by the user. They generally come in a few standard sizes but rather more configurations of actuating lever or button and arrangements of the contacts.
MiniSATA	mSATA (or mini SATA) is a type of expansion card commonly (but not exclusively) used for SSDs, especially in ultra-thin laptops where a conventional 2.5" SSD would be too bulky. See All about SSDs in the Wiki.
Modem	A modem or "modulator/demodulator" is a device used to perform 2-way conversion of a data stream between digital and analogue forms, for example to communicate data over a voice or broadband (analogue) connection.
Modulation	Modulation is the process of impressing an audio or TV signal (for example) onto an RF signal for transmission. See How radios work in the wiki.
MOSFET	A MOSFET or Metal Oxide Field Effect Transistor is a type of field effect transistor where the gate is formed by metal deposited on an extremely thin insulating oxide layer. See Transistors in the Wiki.
Motherboard	The motherboard in a computer (or other piece of electronic equipment) is the main circuit board into which one or more daughter boards can be plugged.
MOV	An MOV or Metal Oxide Varistor is the commonest type of Varistor.
MPP inverter	Getting the most power out of a PV (or solar) cell or wind turbine isn't straightforward. The output voltage will be maximum if you draw hardly any current, and if you draw as much current as you can the output voltage will drop almost to zero. In either case the power delivered (voltage × current) will be minimal. An MPP or Maximum Power Point controller or inverter will continuously adjust the current drawn in order to maximise the power, for example to charge a battery or deliver power to the National Grid.
MPU	A Microprocessor or MPU (MicroProcessor Unit) is the compete central processor of a computer on a single silicon chip, or in the case of a high-end microprocessor, possibly more than one chip in a single package. Additional components are required to provide program and data storage and peripheral functions such as input and output.
mSATA	mSATA (or mini SATA) is a type of expansion card commonly (but not exclusively) used for SSDs, especially in ultra-thin laptops where a conventional 2.5" SSD would be too bulky. See All about SSDs in the Wiki.
MSDOS	MSDOS (Microsoft Disk Operating System, often simply referred to as DOS) was a Microsoft operating system predating Windows, and lacking a graphical user interface.
MTBF	MTBF or Mean Time Between Failures is a measure of the reliability of an item, most usually measured in hours (or thousands of hours).
Multimeter	A multimeter is an electrical test instrument which measures voltage, current, resistance and often several other things. Most usually they have a digital readout but analogue ones with a dial are also available. See How to use a multimeter in the Wiki.
Murphy's Law	Murphy's Law states that if anything can possibly go wrong, it generally will, causing the maximum frustration and inconvenience. A well known example is that if you drop a slice of toast on the floor, it will nearly always fall butter side down. Sod's Law is a slightly more extreme formulation of the same phenomenon, stating that if something can go wrong, it will go wrong. Whereas Murphy's law is of US origin, Sod's Law is thought to have originated in the UK, possibly influenced by the generally inclement and miserable weather frequently suffered on this side of the Atlantic. Whichever of these is more accurate, they constitute absolute proof that we are living in a computer simulation with a bug in the software, since they are in direct contradiction to the laws of probability.
N
N-type	N-type is the name given to a Semiconductor material doped with an impurity whose atoms have one too many electrons to match the crystal lattice. These "spare" electrons can carry an electric current as negative charges.
Neon	Neon is an inert gas present in trace quantities in dry air, but in electronics a "neon" generally refers to a gas discharge lamp giving a characteristic orange glow and much used as an indicator lamp before the advent of LEDs and in signage. Other gasses can be used to give different colours but the lamps using them are still generally referred to as "neons".
NFC	NFC or Near Field Communication is a set of technologies allowing electromagnetic communication between two devices, usually no more than a few centimeters apart. Each device contains a coil of wire and together they form a transformer through which data can be passed. A small amount of power is also often passed to an NFC device such as a security tag, having no power source of its own.
NIC	A NIC or Network Interface Controller is the part of a computer or router that connects it to a network. Formerly it was often a separate add-on card but nowadays it's nearly always built-in to the computer unless an extra one is needed. A computer connected to more than one network or a router has a NIC for each network, and each must have its own IPAddress.
NiCd	NiCd or Nickel Cadmium is a type of rechargeable battery, similar in many ways to Nickel Metal Hydride but with higher maximum charge and discharge rates making it suitable for power tools and radio controlled models, and having greater tolerance to abuse, but suffering from a "memory effect" and containing toxic cadmium. See Batteries in the Wiki.
NiMH	NiMH or Nickel Metal Hydride is a type of rechargeable battery, similar in many ways to Nickel Cadmium but with greater capacity, reduced "memory effect" and containing no toxic cadmium, but having a higher self-discharge rate. See Batteries in the Wiki.
Noise	Noise, in the context of electronics and communications, is any unwanted interference tending to mask a desired signal. It can never be eliminated entirely as the electronics we're surrounded with all generates a certain amount of interference, and an electric current itself consists of a random flow of electrons.
NPN	A type of junction transistor comprising a P-type layer sandwiched between two N-type layers.
NTC Resistor	A negative temperature coefficient or NTC resistor is a resistor whose resistance falls strongly with increasing temperature, unlike most resistors whose resistance increases slightly with temperature. Used as a temperature sensing component in electronic thermometers, to warn of overheating, particularly in lithium batteries, and as an inrush current limiter.
NVMe	NVMe (Non-Volatile Memory Express) is an interface standard and logical access method optimised for accessing SSDs. PCIe is commonly used as the physical interface.
O
Object Code	Software in a form that can be directly executed by a computer, but is virtually unintelligible to humans, simply comprising binary numbers.
Octal	A number system based on only 8 digits 0 - 7. This is useful because each octal digit can represent 3 bits. See also International Octal (a type of valve base).
Octet	A byte or octet is 8 bits, which can be used to represent a single letter, number or punctuation mark, or a number between 0 and 255, or anything else that can have up to 256 values.
OEM	Original Equipment Manufacturer. A somewhat confusing term generally meaning a company which sells another company's manufactured goods under its own branding and with its own warranty. Also used of software, for a licence allowing a computer manufacturer to install the software on systems they build. (In fact, the manufacturer could be you, building a computer from standard parts.)
Ohm	A bit like friction, the Ohm is the unit of resistance to flow of electricity. Named after German mathematician and physicist Georg Simon Ohm, chiefly remembered for discovering Ohm's Law. See Electric circuits, volts amps watts and ohms in the Wiki.
Ohm's Law	The amount of current (Amps) flowing in a circuit is the pressure (Volts) divided by the resistance (Ohms), or I = V / R.
OLED	Organic Light Emitting Diode. A light emitting diode using an organic (i.e. carbon-based chemical) semiconductor rather than one based on inorganic materials such as silicon or a compound of gallium. Also used to refer to a display made up of many OLEDs.
Open Circuit	Electricity likes to go all the way round a circuit and back to where it started. When there is a break in the circuit preventing the current from flowing (maybe due to a fault) this is termed an open circuit.
Open Source	Software or hardware the source code or design of which is non-proprietary and made freely available for anyone to inspect or modify for their own purposes.
Operating System	The software which manages the computing, storage and communications resources of a computer, and presents a user interface allowing the user to run and interact with programs as desired.
Operational Amplifier	An operational amplifier (commonly known as an op amp) is a high gain amplifier, usually an integrated circuit. With a feedback loop it can be used to construct a variety of circuit functions such as high quality amplifiers and filters.
Opto coupler	An LED and a phototransistor, electrically isolated but in a single packaged. Used for signalling between two circuits which must have no electrical connection between them.
Oscillator	A circuit which produces an AC output at a chosen frequency.
Oscilloscope	An oscilloscope (often referred to simply as a "scope") is a device for producing a graphical representation of how an electrical signal varies with time.
Own	In hacker slang, to gain full control of a device or computer through subversive means. See also Pwn.
P
P-type	P-type is the name given to a Semiconductor material doped with an impurity whose atoms have one too few electrons to match the crystal lattice. These result in holes which can effectively move about and carry an electric current as positive charges. Think of bubbles in a liquid. These "holes" in the liquid effectively have a negative weight and so tend to rise against gravity.
PAN	A PAN is a Personal Area Network, i.e. a computer network linking a single person's devices, typically covering, at most, a single room.
Parallel	Two elements of a circuit are said to be in parallel if they are connected so that the current can equally pass through one or the other. For example, if you connect a battery to a bulb through two switches in parallel, the bulb will light if either or both switches are switched on.
Parallel Interface	A parallel interface is a data connection in which some number of bits (for example the 8 bits or a byte, or maybe 16 or 32 bits) are all sent simultaneously down separate wires. Whilst this can be a fast method of transmission, the speed and physical length of the connection are limited by the fact that slight differences in the path lengths may mean that not all bits arrive quite together.
Partition	A partition is a subdivision of a disk. Under the control of the operating system, a disk can be sliced up like a cake into several such partitions, each being dedicated to a particular purpose. Otherwise, you would have to install another physical disk for each purpose. Also, large computers are sometimes made to pretend to be several independent smaller ones, and these can also be referred to as partitions.
Parts pairing	Parts pairing or serialisation is the practice of some manufacturers of assigning unique serial numbers to subassemblies of a device, such as a smartphone screen, and preventing full functionality of a replacement unless it has been paired with the device through a process not available to independent repairers.
Passkey	Passkeys are a more secure and more convenient alternative to passwords for logging into a website. Your browser creates and stores a highly secure secret key and gives the website a corresponding public key. Using the public key and clever cryptographic magic, the website can subsequently prove that your bowser knows the secret key without your browser ever revealing what it is. A limitation is that you can only share passkeys between different platforms (Windows/Apple/Adroid/Linux) or different browsers if you entrust them to a cross-platform password manager.
Password Manager	A password manager is a program or app which stores all your passwords in an encrypted vault under a (hopefully) strong master password. Many password managers will also generate and remember totally random unguessable passwords, and automatically apply them to login password prompts.
Password spraying	A password spraying attack is when an attacker tries a small number of common passwords against many different accounts on a system in the (often justified) hope that one of them, at least, will be using a weak password.
PAT Test	A PAT or Portable Appliance Test is a basic electrical safety test which should be applied to all mains operated electrical equipment before and after attempting a repair. It comprises a visual inspection and an automated test. See PAT testing in the Wiki.
PATA	PATA or Parallel ATA is an old parallel interface used for connecting storage devices to a computer. It has been superseded by SATA but may still be seen in old computers up to the early years of the millennium.
PCB	A PCB or printed circuit board is a board on which conductive copper tracks are used to make the connections between the components, rather than individual wires.
PCI	PCI or Peripheral Component Interconnect refers to a type of parallel computer bus and the expansion ports attached to it common in desktop computers in the late 90's and early 2000's. It has been superseded by PCIe and the original PCI has been renamed Conventional PCI.
PCIe	PCIe or PCI Express is a serial computer bus superseding conventional PCI and used for connecting peripheral devices of any kind to a computer.
PD	PD or Potential Difference is a difference of voltage between two points in a circuit.
PEBKAC	Problem Exists Between Keyboard And Chair (or a variant) is an expression sometimes used by computer experts and support staff to indicate that a problem lies with a user rather than the computer system. It should be used sparingly and with caution as it can cause offence, also bearing in mind that the real source of the problem might often be shortcomings in the usability of the system or opaque documentation.
Peltier	The Peltier effect is a thermo-electric effect by which a junction between two different conductors or semiconductors warms if a current is passed through it in one direction and cools if it is passed in the opposite direction. The converse of the Seebeck effect. Used for cooling in mini fridges.
Pentalobe	Pentalobe is a type of tamper-resistant screw and screwdriver introduced by Apple. See Types of screw head and screwdriver in the Wiki.
Pentode	A pentode is a valve with 5 electrodes: an anode, a cathode and 3 grids: a control grid (as in a triode), a screen grid to reduce stray interaction between the control grid and the anode, and a suppressor grid which ensures that electrons hitting the anode stay there.
Percussive maintenance	Percussive maintenance is a fixing technique often employing a Birmingham screwdriver and requiring great skill in its application. A sharp blow in precisely the right location can sometimes resolve the most inscrutable of faults. Mis-applied, however, it can only make things worse.
Peripheral	In computing terms, a "peripheral" is any device which allows applications running on the central processor to interact with the real world. This can include monitors, keyboards, mice, speakers, printers, scanners, cameras and network interfaces, but equally in an industrial setting, might include all manner of sensors, actuators and motors.
PGA	PGA or Pin Grid Array is a type of integrated circuit package with an array of pins on the underside, and fitting in a zero insertion force connector. It's commonly used for processor chips on a motherboard, allowing a choice of processor to be fitted according to customer requirements.
Phase-locked loop	A phase locked loop or PLL is a circuit in which both the frequency and phase of a variable frequency oscillator is locked to a reference frequency or to a frequency with a simple numerical relationship to it. The many applications include digital tuning in a radio and the generation of the different clock frequencies for the processor and RAM in a computer.
Phillips	Phillips is a type of cross-head screw or screwdriver, similar to but not quite the same as Pozidriv. See Types of screw head and screwdriver in the Wiki.
Phishing	Phishing is the criminal practice of sending emails or messages through social media inviting the recipient to open a malicious attachment or visit a malicious website.
Phono plug	A phono or RCA connector is a type of coaxial connector commonly found in audio and video equipment. It is characterised by a central pin which protrudes from the cylindrical shell. Different colours are used to denote different audio or video applications.
Photon	A photon is a particle of light. Light has both wave-like and particle-like properties as described by Quantum Mechanics. If you don't understand that, don't worry. Nobody understands Quantum Mechanics.
Phototransistor	A phototransistor is a light-sensitive transistor used for sensing light.
Piezo	The piezo-electric effect is the property of some materials to develop a voltage across two opposite faces when a physical force is applied, or to stretch or squeeze slightly if a voltage is applied. Pronounced "pee-ayzo" by some and "pee-zo" by others. The Oxford Dictionary allows either, but wars have been started over less contentious issues. Used in some microphones and earpieces, quartz crystal frequency standards, and gas lighters.
Pig butchering scam	A "pig butchering" scam is a particularly insidious type of fraud in which a fraudster builds trust with the victim over a period of time, then persuades them to contribute increasing sums, typically to a fake cryptocurrency investment. Once a substantial sum has been collected or when the victim tries to withdraw their "investment", the fraudster becomes uncontactable, leaving the victim both financially ruined and emotionally devastated. Southeast Asian "fraud factories" have become a prolific source of such scams, manned by forced labour from human trafficking. Wikipeda has more details.
Pin	The connections to an IC (or other component) are universally known as "pins", whether or not they physically resemble a pin, as they do (arguably) in older through-hole devices.
Pinout	The pinout is a term used for the definition (as a list or as a diagram) of the connections to each of the pins of a plug, a socket or an electronic component such as an integrated circuit with multiple connections.
PIR	A PIR or Passive Infrared detector is a device which detects heat radiation from warm bodies. Often used to turn on an outdoor security light, or to allow a room light to be turned off automatically when no one is there.
Pixel	A pixel is an individual picture element, or coloured (or black and white) dot, thousands or millions of which make up a digital image.
PLC	A PLC or Programmable Logic Controller is a device much used in industry for controlling plant and machinery. It has a number of inputs which would be connected to sensors of different kinds, and outputs through which it can control switches, valves, motors, pumps etc in order for the machine or industrial process to operate correctly and safely. Their pre-electronic origins lie in relay logic and they are still normally programmed by similar ladder logic as their predecessors.
PLL	A phase locked loop or PLL is a circuit in which both the frequency and phase of a variable frequency oscillator is locked to a reference frequency or to a frequency with a simple numerical relationship to it. The many applications include digital tuning in a radio and the generation of the different clock frequencies for the processor and RAM in a computer.
PNP	PNP is a type of junction transistor comprising a thin N-type layer sandwiched between two P-type layers.
Polyfuse	A polyfuse or polyswitch or polymeric positive temperature coefficient (PPTC) device is an alternative to a conventional fuse for protection against a dangerously large current flow in a fault condition. It consists of a polymer loaded with carbon particles. An excessive current causes it to heat up and the polymer to expand so that the carbon particles are no longer in contact. As a result, the current can no longer flow until it's allowed to cool down. See Polyfuses in the Wiki.
POP3	If you manage your email using a program or app on your own device (rather than using webmail) then POP3 or Post Office Protocol 3 provides a method by which your program or app can retrieve incoming emails from an email server for storage on your device.
Port	A TCP or UDP port is simply a number designating a particuar service on a server. For example connecting to a computer with port number 80, the connection will be routed to the computer's web server software, or if you want an encrypted connection you connect to TCP port 443. Many other services for example email, DNS, time services, file and print services are distinguished by well known port numbers.
Pot	A potentiometer (commonly known as a pot) is a resistor with a slider that can be moved along its length from one end to the other to tap off any required amount of resistance, for example for a volume control. See Potentiometer in the Wiki.
Potentiometer	A potentiometer (commonly known as a pot) is a resistor with a slider that can be moved along its length from one end to the other to tap off any required amount of resistance, for example for a volume control. See Potentiometer in the Wiki.
POTS	POTS or Plain Old Telephone System refers to the legacy analogue telephone system carried over copper wires.
Power Supply	A PSU or Power Supply Unit is that part of a piece of equipment which takes the raw input power, for example from a battery or from the mains, conditions it, and delivers it at the voltage(s) the equipment needs internally.
Pozidriv	Pozidriv is a type of cross-head screw or screwdriver, similar to but not quite the same as Phillips. See Types of screw head and screwdriver in the Wiki.
PPE	PPE or Personal Protective Equipment is equipment such as safety glasses, dust and fumes extractors, respirators, ear plugs, protective clothing etc. used for protection against health and safety hazards.
PPTC	A polyfuse or polyswitch or polymeric positive temperature coefficient (PPTC) device is an alternative to a conventional fuse for protection against a dangerously large current flow in a fault condition. It consists of a polymer loaded with carbon particles. An excessive current causes it to heat up and the polymer to expand so that the carbon particles are no longer in contact. As a result, the current can no longer flow until it's allowed to cool down. See Polyfuses in the Wiki.
Primary Battery	A primary battery is one which cannot be recharged but has to be thrown away (or recycled) once the chemicals it contains are exhausted.
PrivEsc	An Escalation of Privilege (or EoP or PrivEsc) vulnerability is a software flaw which allows an attacker, already operating on your computer, to increase the privileges allowed to him so as to gain full control.
Privilege Escalation	An Escalation of Privilege (or EoP or PrivEsc) vulnerability is a software flaw which allows an attacker, already operating on your computer, to increase the privileges allowed to him so as to gain full control.
Protocol	A protocol in computing terms is a set of rules or standards defining the way in which data is to be exchanged between two or more computers or components of a single computer, for example what messages can be sent and in what contexts by each party, the permissible responses, and how those messages and responses are to be structured and interpreted. Think of it as a Highway Code for computers.
PSTN	PSTN or Public Switched Telephone Network is the global switched telephone network. Unlike the Internet in which data is transmitted as independant packets, connections between subscribers are made and taken down as required.
PSU	A PSU or Power Supply Unit is that part of a piece of equipment which takes the raw input power, for example from a battery or from the mains, conditions it, and delivers it at the voltage(s) the equipment needs internally.
Public key cryptography	Public key or asymmetric cryptography is a form of encryption using separate keys for encryption and decryption, neither of which can be derived from the other. Anyone can use the public key to encrypt a message which only the holder of the corresponding private key can decrypt. Alternatively, the holder of a private key can "digitally sign" a message by using it to encrypt a digest of the message, sent with the message itself. A recipient can prove the authorship of the message by successfully decrypting the digest using the author's previously published public key. Public key cryptography is computationally expensive so is often used simply to agree a symmetric encryption key.
Pulse Width Modulation	PWM or Pulse Width Modulation is a highly efficient method of power control in which the power is switched on and off very rapidly. The output is then smoothed with a capacitor and/or inductor and the relative on and off times then determine the power that is delivered.
PUP	A PUP or Potentially Unwanted Program is any useless but generally (though not necessarily) harmless program, often installed by the computer vendor or (knowingly or unknowingly) installed along with downloaded software. The main purpose of it is usually to sell you some other software which you probably don't want.
PV Cell	A PV or Photo Voltaic Cell (commonly known as a solar cell) is a device for converting light into electricity.
PWM	PWM or Pulse Width Modulation is a highly efficient method of power control in which the power is switched on and off very rapidly. The output is then smoothed with a capacitor and/or inductor and the relative on and off times then determine the power that is delivered.
Pwn	In hacker slang, to pwn (pronounced "pone") a device or computer is to hack and comprehensively take control of or "own" it. The term derives from a mis-typing of "own" in a video game in the 1990's.
Q
Q factor	The Q (or quality) factor of a resonant system, such as a tuned circuit, a quartz crystal, a guitar string or a pendulam, is a measure of how free it is to resonate at its natural frequency. It can be defined in terms of the sharpness of the resonant frequency, or as the number of times greater the stored energy of the resonator is than the energy lost per vibration or cycle. For example, a tuning fork which rings for a long time after being struck, is exhibiting a high Q due to minimal losses.
Quantum Computer	Don't worry, you may never have to fix one of these. One of the predictions of Quantum Mechanics is that sub-atomic particles can effectively be in more than one state at a time. In theory, you should be able to use this property to build a computer that explores many possible solutions to a problem simultaneously rather than trying them one by one, as a normal computer would. This could be really neat for cracking cryptographic codes, or working out the shortest route by which you could visit all the pubs in your neighbourhood. But don't hold your breath: the best a quantum computer has managed so far is to factorise 33 into 3 times 11.
Quantum Mechanics	Quantum Mechanics (or QM for short) is the counter-intuitive theory of how things behave at the atomic and sub-atomic levels. It has passed every test physicists have thrown at it, yet nobody understands it, only how to use its predictions to design lasers and silicon chips and all the modern gadgets that depend on them.
Quartz crystal	A quartz crystal is an electronic component containing a piece of quartz cut and polished so as to resonate at a very specific frequency. Usually housed in a silvery metal can, it's very easy to spot on a circuit board. See Crystals and Ceramic Resonators in the Wiki.
Qubit	A qubit is a quantum bit which can be in two different states simultaneously. We have never seen one of these at a Restart Party, and if we ever do we will probably be undecided about what to do with it.
R
R2R	R2R or Right to Repair is the movement and campaign to make repair options widely available at reasonable cost, and to outlaw practices which unnecessarily make repair uneconomic, difficult or virtually impossible. Such practices include non-availability or exorbitant cost of spare parts or service information, manufacturing methods making disassembly difficult, and parts pairing or software locks restricting repair to the manufacturer or their authorised agents. These practices are commonly justified by often spurious arguments relating to maintaining quality and security.
RAM	RAM or Random Access Memory is the very fast memory used by a computer as working store for the programs and data currently being processed.
Ransomware	Ransomware is a particularly nasty form of Malware which encrypts all your files then demands a ransom in order to recover them for you. But there is no guarantee that you'll get you're data back even if you pay the ransom.
RAT	A RAT or Remote Access Trojan is a type of Malware which allows a remote attacker to see your screen, to move and click your mouse and to enter keystrokes on your keyboard, just as if he were sitting in your chair.
Razor-and-blades business model	The razor-and-blades business model is the pricing strategy in which a basic commodity is sold cheaply but then needs to be serviced by expensive consumables. Current examples include printers and printer ink, coffee machines and coffee pods, games consoles and games, and mobile phones and usage contracts. The model is often attributed to Gillette, inventor of the safety razor, but was in fact introduced by his competitors.
RCA plug	A phono or RCA connector is a type of coaxial connector commonly found in audio and video equipment. It is characterised by a central pin which protrudes from the cylindrical shell. Different colours are used to denote different audio or video applications.
RCE	A Remote Code execution or RCE vulnerability is a software flaw which allows a remote attacker (probably somewhere else on the Internet) to execute arbitrary code of his choosing on your computer, usually as the first step in taking it over.
Rectifier	A rectifier consists of a diode or 4 diodes connected together to convert AC to DC.
Recycling	Recycling is the practice of recovering raw materials from waste items. This may be inefficient, and impracticable or uneconomic for valuable materials used in small quantities. Furthermore, since the resultant raw materials may not be of sufficient quality for demanding applications, in many cases it is better described as downcycling. Hence it is regarded as the second-worst option, ahead only of landfill or incineration.
Reed switch	A reed switch is a sealed glass tube containing two magnetic contacts forming a switch. The switch can be closed by bringing a magnet near or by energising an electromagnet. It can be used as the switch in a reed relay or with a magnet attached to a moving part, for example to detect when a cover is closed, a printer paper drawer is fully inserted, or a float in a tank is indicating that the tank is full.
Refurbishment	Reurbishment is the practice of taking used items and giving them a new lease of life by replacing worn or damaged parts, bringing them up to date with a fresh software installation and perhaps replacing a slow hard drive with a much faster SSD.
Relay	A relay is a device consisting of a switch or a bank of switches operated by an electromagnet. The switch(es) may be simple switch contacts or a reed switch.
Reluctance	When a magnetic material such as iron is subject to a magnetic field, the atoms tend to line up with the field, increasing its effect. Magnetic reluctance is a measure of how much magnetic field is required to produce a given magnetic effect. Fresh air is non-magnetic and so has a high reluctance, unlike iron, which is very susceptible to magnetism.
Reluctance motor	A reluctance motor is one in which coils in the stator produce a rotating magnetic field which drags a shaped magnetic rotor around with it as it continually tries to keep aligned with the direction of the rotating magnetic field.
Remanufacture	Remanufacture is the practice of creating good-as-new products using some proportion of pre-used parts. Common examples are ink and toner cartridges and laptop batteries, could equally include a laptop given a new case, keyboard and battery.
Remote Code Execution	A Remote Code execution or RCE vulnerability is a software flaw which allows a remote attacker (probably somewhere else on the Internet) to execute arbitrary code of his choosing on your computer, usually as the first step in taking it over.
Repair	Repair is the second option (after reuse) in a circular economy. It has been shown that many devices sent for recycling could actually be repaired relatively easily and hence given a new lease of life.
Repair Café	Repair Café International is an organisation based in Amsterdam and run by Martine Postma, who developed the original concept for Repair Café events and sparked the community repair movement. Repair Cafés are events at which volunteers help members of the public repair their electrical, electronic, mechanical and clothing items. Since the first ever repair café in 2009, Repair Café International now represents a global grassroots movement promoting repair through community events. It has much in common with The Restart Project but is a separate organisation with slightly different emphasis and aims.
Reservoir Capacitor	A reservoir or smoothing capacitor is a large value capacitor used for smoothing rectified AC. See power supplies in the wiki.
Resistance	Resistance is the property of resisting the flow of an electric current, much like how mechanical friction resists motion. It is measured in Ohms.
Resistor	A resistor is an electronic component generally containing a thin metal or oxide film or a thin wire through which electricity flows with difficulty. It's resistance is measured in Ohms. A resistor always generates heat, even if only a tiny amount, and any replacement must be rated in Watts accordingly. See Resistors in the Wiki.
Restart Project	The Restart Project is a people-powered social enterprise based in London (UK) that aims to fix our relationship with electronics. Since 2012, it has run "restart parties" at which skilled amateur fixers could help members of the public repair their broken electrical and electronic gadgets. It has since gone global, supporting communities and networks of repair around the world, as well as assuming a leading role in campaigning for Right to Repair and related issues. Whilst it shares many aims with Repair Café International, it is a separate organisation.
Reuse	Reuse is the preferred option in a circular economy. Many items are discarded because they have been replaced by a newer model, or simply fallen into disuse, but could be given a second life if sold or donated to a new owner.
Reverse defenestration	Reverse defenestration is a term used to describe the process of throwing Windows out of computers. This is generally in order to install Linux or some other non-Microsoft operating system instead. In partial reverse defenestration, a Windows installation may remain in a dual-boot configuration or as a virtual machine but this is not admitted as true reverse defenestration by purists, who prefer radical reverse defenestration, leaving behind no vestiges of Windows.
RF	RF or Radio Frequency is a high frequency capable of being transmitted as a radio signal.
RFID	RFID or Radio Frequency Identification is the technology by which a passive identification tag can be interrogated by a reader by energising it at short range with an electromagnetic field, then detecting a response.
RGB	RGB refers to the separate red, green and blue components of a colour image or video.
Ribbon cable	Ribbon cables are cables with the conductors laid side by side to form a ribbon, rather than bound together in a bunch. The conductors may be individual insulated wires or may be copper tracks like those on a printed circuit board, laid on a flexible substrate. They are normally used in conjunction with special connectors.
Right to Repair	R2R or Right to Repair is the movement and campaign to make repair options widely available at reasonable cost, and to outlaw practices which unnecessarily make repair uneconomic, difficult or virtually impossible. Such practices include non-availability or exorbitant cost of spare parts or service information, manufacturing methods making disassembly difficult, and parts pairing or software locks restricting repair to the manufacturer or their authorised agents. These practices are commonly justified by often spurious arguments relating to maintaining quality and security.
Ripple	Ripple is the name given to the residual variation after smoothing in the output voltage of a DC power supply. It occurs at the frequency or twice the frequency of the AC mains input.
RISC	RISC or Reduced Instruction Set Computer is a computer architecture in which the basic machine instructions are very simple and so can be implemented cheaply. Although more instructions may be needed for a given task, they can be made to run very fast and the silicon saved can be used for other optimisations such as large cache memories to speed up execution. The result can be a significant net gain in performance. ARM is the most widely used example of this architecture.
RISC	RISC or Reduced Instruction Set Computer is a computer architecture in which the basic machine instructions are very simple and so can be implemented cheaply. Although more instructions may be needed for a given task, they can be made to run very fast and the silicon saved can be used for other optimisations such as large cache memories to speed up execution. The result can be a significant net gain in performance. ARM is the most widely used example of this architecture.
RISC-V	RISC-V (pronounced Risk Five) is a RISC computer architecture with its roots in an academic design at Berkley. There are 32 and 64 bit variants of the basic design and numerous optional extensions giving additional instructions for different data types and types of operation. Unlike ARM, the design is completely open source.
RJ11	RJ11 is a type of connector commonly used with telephones. It has up to 4 connections and a kind of barb which prevents it from being accidentally pulled out of a matching socket.
RJ45	RJ45 is a type of connector commonly used on network cables. It has 8 connections and a kind of barb which prevents it from being accidentally pulled out of a matching socket.
RMS	Since AC is continually varying it can't be measured as a unique voltage. The RMS or Root Mean Square value (a mathematical construct) is the DC voltage that would deliver the same power into a given load.
RoHS	The Restriction of Hazardous Substances or RoHS directive (sometimes known as the "lead-free directive") restricts the use of 10 hazardous substances in electrical and electronic equipment. These include lead, mercury, cadmium and hexavalent chromium, together with a further six organic chemicals. It took effect in the EU in 2006.
ROM	ROM or Read-only memory is memory which can be read as often as you like but cannot (or cannot so easily) be changed. It is typically used for the firmware of a device.
Robertson	Robertson is a type of screw and screwdriver common in Canada but used elsewhere, especially in woodworking and general construction. The screw head has a square depression, slightly tapered to facilitate the insertion of a matching screwdriver with a square tip.
Rootkit	A rootkit is an especially pernicious type of malware which masks its presence and hinders its removal by subverting the operating system in such a way as to expunge all references to itself and associated malware in listings of files and running programs.
Rotary encoder	There are many types of rotary encoder, but simple mechanical ones are often found behind the volume control in a digital radio. It can be turned as far as you like in either direction, but in a series of steps. At each step, one of two switches within it turns on or off. This allows a microcontroller to detect the direction and extent to which it's tuned.
Rotor	The rotor in an electric motor is the part that rotates, as opposed to the stator, which is the part that doesn't. More generally, a rotor can be any part of a machine which rotates.
Router	A router is a network device connecting two or more networks. It forwards packets of data received from one network to its destination or to another router closer to its destination on another network.
RS-232	RS-232 is an old standard originally conceived for serial data communication between a modem and a computer or computer terminal, normally using DB-9 or DB-25 connectors. It was embodied in the COM ports that were once a feature of virtually all personal and home computers. Although now superseded by USB, due to its great simplicity and former ubiquity it is still used where low speed communication is required over a limited distance. RS-422 and RS-485 are later variants offering enhance speed, maximum distance or connnectivity options.
RTFM	Read The Fine/Frigging/Flippin'/eFfing Manual, a comment sometimes aimed at oneself, when it becomes apparent or when the person addressed perhaps should have realised that the answer to a problem is plainly covered in the manual, handbook or other documentation. Except in a self-deprecating sense, it should be used sparingly and with caution as it can easily cause offence.
S
S/N Ratio	The Signal to Noise (or S/N) ratio is a measure of the strength of a signal compared to any background noise or interference, and is measured in decibels (dB). In an audio system, a high S/N ratio tells you that any background hiss will be imperceptible, and in data communications, the S/N ratio directly affects the rate at which you can transmit data.
S/PDIF	S/PDIF (Sony/Philips Digital Interface) is a standard for encoding digital audio for transmission using optical fibre using TOSLINK connectors or coaxial cable using phono or BNC connectors.
Sandbox	A sandbox is a kind of software "padded cell" in which untested or untrusted software can be tried out without risk to the host computer, a bit like a sandbox in which a child can play safely with the sand.
SATA	SATA or Serial ATA is a serial interface used for connecting storage devices to a computer. It supersedes the older PATA.
Saturation (colour)	The saturation of a colour is its purity. This can be used together with the chrominance (the hue) and the luminance (the brightness) as an alternative to specifying the red, green and blue (RGB) components. This is useful because the eye perceives variations of chrominance and saturation across an image much less sharply than variations of luminance. Hence these latter can be presented at lower definition with no visible degradation in the image, so saving storage or transmission bandwidth. It also allows a graphic designer to select a range of related colours by varying just one of those three parameters.
Saturation (magnetic)	A magnetic material such as iron is said to be saturated when it cannot be magnetised any more. This is undesirable in a transformer or an inductor as its performance is then reduced. But in digital magnetic recording (in a hard disk or a digital tape) the surface is deliberately saturated in order to reliably record ones and zeros.
Saturation (transistor)	When a junction transistor used as a switch in a digital circuit is turned on as hard as it will go it is said to be saturated. In this condition there is excess charge in the collector region, and this has to drain away before the transistor will switch off again. This limits the speed at which it can switch, and so saturation is a condition designers try to avoid.
Schematic	A schematic is another name for a Circuit Diagram.
Scope	An oscilloscope (often referred to simply as a "scope") is a device for producing a graphical representation of how an electrical signal varies with time.
SCR	A thyristor, SCR or Silicon Controlled Rectifier is an electronic component with 3 leads used as a switch. See Transistors in the Wiki.
SDR	An SDR or Software Defined Radio is a radio where the wanted signal is extracted by software, or in the case of a transmitter, the signal is generated by software. Thus, one radio can perform many different functions just by changing the software, whether it be sound or television broadcast, or data such as WiFi, Bluetooth or GPS. See Software Defined Radio in the Wiki.
Secondary Battery	A secondary battery is one which can be recharged, restoring the chemicals it contains to their original state (or nearly).
Sector	A sector is the smallest block of data a disk can read or write. On older disks this was normally 512 bytes but in modern disks and SSDs it is usually 4096 bytes.
Seebeck	The Seebeck effect is a thermo-electric effect by which a voltage is generated by a junction between two different conductors or semiconductors if it is heated or cooled relative to the rest of the circuit. The converse of the Peltier effect. Used by space craft venturing to the outer reaches of the solar system to generate electricity from a radioactive heat source such as a lump of plutonium.
Semiconductor	A semiconductor is a material such as silicon, germanium or gallium arsenide which can be made to conduct electricity either by negatively charged electrons (N-type) or by positively charged electron vacancies, or holes (P-type), by adding small amounts of carefully chosen impurities.
Serialisation	Parts pairing or serialisation is the practice of some manufacturers of assigning unique serial numbers to subassemblies of a device, such as a smartphone screen, and preventing full functionality of a replacement unless it has been paired with the device through a process not available to independent repairers.
Serial Interface	A serial interface is a data connection in which the bits are all sent one after another down the same wire. Consequently they can be sent very fast indeed and use simple connectors with few pins. The COM ports found on old computers, USB on modern ones and SATA are all examples of serial interfaces.
Series	Two elements of a circuit are said to be in series if they are connected so that the current has to pass through one then the other. For example, if you connect a battery to a bulb through two switches in series, the bulb will only light if both switches are switched on.
Server	A server in computer terms is a computer (or a program running on a computer) which offers storage or processing services to a client computer (or many such), the latter dealing with user interaction.
SES	SES or Small Edison Screw is a smaller version of the ES screw-fitting lamp base. See Types of bulb in the Wiki.
Shell	A shell is a program which offers a user interface to lower level parts of an operating system (referred to as the kernel), which themselves lack a user interface. The Mac or Linux command prompt is referred to as the shell, and the Windows Desktop is Windows' shell.
Short circuit	If two wires are touching which shouldn't the current may be able to take a short cut, meeting very little resistance and so allowing too much to flow. This is a Bad Thing and can cause your gadget to get hot or even catch fire. A short circuit can also be caused by an inadvertent solder bridge between two tracks on a printed circuit board, and this may simply cause the device to malfunction.
Shottky diode	A Shottky diode is a type of diode using a metal-semiconductor junction, mainly used in power supplies on account of its ability to operate efficiently at higher frequencies. See switching mode power supplies in the wiki.
SiC	Silicon carbide (SiC), as carborundum, has long been used as an abrasive, but it is also a semiconductor with useful properties for high power and high voltage applications. In recent years silicon carbide devices such as MOSFETs offering superior specifications to equivalent silicon devices have been increasingly used in applications such as EVs and power conversion.
Sicilian screwdriver	A sicilian screwdriver is a cocktail made with blood orange juice and vodka, especially refreshing after an arduous session of fixing. Full details are given in the Wiki.
SIDAC	A SIDAC or Silicon Diode for AC is a 2-terminal electronic component comprising 5 semiconductor layers which abruptly switches from a non-conducting to a conducting state when a certain voltage is applied, typically in the region of 150 - 300V depending on the type. It then remains conducting until the current falls below a small holding current. It is similar in operation to a DIAC except for the higher threshold voltage. It has a few niche applications in air ionisers and starting circuits for metal halide and other high pressure vapour lamps.
Side channel attack	A side channel or covert channel attack is one which exploits some side-effect of the processing, transmission or storage of data in order to draw inferences about the data itself. For example, unless carefully designed, variations in the power consumed by a processor while encrypting data can leak information about the data, perhaps ultimately leading to discovery of the encryption key. However, the rate of data leakage is usually very low, and insignificant in all but the most secure environments.
Signal to Noise Ratio	The Signal to Noise (or S/N) ratio is a measure of the strength of a signal compared to any background noise or interference, and is measured in decibels (dB). In an audio system, a high S/N ratio tells you that any background hiss will be imperceptible, and in data communications, the S/N ratio directly affects the rate at which you can transmit data.
Silicon	Silicon is the semiconductor material used in the vast majority of transistors and integrated circuits. It almost entirely superseded germanium in the 1960's but gallium arsenide if now often used for very high frequencies and silicon carbide and gallium nitride are taking over for high voltages and powers such as in electric vehicles.
Silicon Carbide	Silicon carbide (SiC), as carborundum, has long been used as an abrasive, but it is also a semiconductor with useful properties for high power and high voltage applications. In recent years silicon carbide devices such as MOSFETs offering superior specifications to equivalent silicon devices have been increasingly used in applications such as EVs and power conversion.
SIL	SIL or Single In-Line is a type of packaging used for circuit sub-assemblies, resistor networks and some ICs, and having a single row of pins designed for through-hole assembly.
SIM swap	A SIM-swap attack is when a criminal convinces your mobile provider to give them a new SIM card for your phone. When they activate it your phone will go dead and they will then be able to request SMS-based password resets for your email and other online accounts, all of which will now go to their phone instead of yours. This is going to be a very bad day for you.
Simplex	A simplex communication channel is one that allows for commuication in one direction only, in conttrat to a duplex connection which supports 2-way communication.
Sine wave	A sine wave is the mathematically simplest form of vibration or oscillation, which can be represented by a single frequency only with no harmonics or overtones. Exemplified by the motion of a pendulum swinging through a small angle, or the height of a point on a wheel above or below the axle as the wheel turns at constant speed.
S.M.A.R.T	S.M.A.R.T or Self-Monitoring, Analysis, and Reporting Technology is a technology built into all modern hard disks and solid state disks by which the device can monitor things like its error rate and report back, hopefully giving early warning of impending failure.
Smartphone	A smartphone is generally taken to mean a full-featured mobile phone with a graphical touch screen and offering (in addition to making and receiving voice calls) email, instant messaging, web browsing and a wide range of additional functions provided through downloadable apps.
SMD	A Surface Mount Device or SMD is an electronic component designed to be mounted on the surface of a printed circuit board, instead of having leads which pass through holes in the board, soldered on the reverse side. Surface mount devices are generally smaller, often much smaller than their through-hole equivalents.
Smidgeon	A smidgeon is the size of small adjustment made with the benefit of much experience, generally rather more than a gnat’s whisker and definitely less than a tad.
Smoothing	Smoothing is the process of turning raw rectified AC (which drops to zero twice in every cycle of the AC) into a reasonably constant voltage. See Why is a power supply needed? in the Wiki.
SMPS	An SMPS or Switch-Mode Power Supply is a power supply which uses a switch-mode regulator, generally in conjunction with a small ferrite-core transformer, rather than a heavy and bulky iron-core transformer and linear regulator.
SMR	SMR or Shingled Magnetic Recording (as used in an SMR drive) is a method of increasing the recording density on a hard drive by writing each track so as to partially overlap the previous one, in the manner of overlapping roof tiles (or shingles). This means that previously written data cannot be overwritten as it would partially overwrite both the previous and following data tracks. If data is updated it must be appended to the existing data, and either the disk controller or the operating system must keep track of its new location on the disk, and also remember that the old copy is now stale. Once a contiguous block of data of a certain size has been marked as stale, it can all be overwritten from the start of the block with more shingled data tracks. SMR drives are best suited to applications such as archivng, where data is rarely updated, or mostly updated in large chunks.
SMTP	If you manage your email using a program or app on your own device (rather than using webmail) then SMTP or Simple Mail Transfer Protocol is the method by which your program or app submits outgoing email to a mail server.
SNAFU	Situation Normal, All Fouled Up is a sarcastic expression, believed to have originated in the United States Marine Corps during World War II, describing a bad yet not unfamilar situation. A FUBAR is, if anything, somewhat worse.
Snubber	A snubber is a circuit element designed to absorb the voltage spike which tends to be created when the current is abruptly switched off through an inductive load such as an electric motor, so protecting other parts of the circuit or other devices powered by the same source.
Soak test	A soak test is an extended test of a device following manufacture or repair, with the aim of shaking out any manufacturing defects or shortcomings in the repair. Sometimes this will be performed at full power or in some other way designed to stress any weak components and precipitate any incipient failures.
SoC	An SoC or System-on-Chip is a complex integrated circuit comprised of a number of standard building blocks making up a complete self-contained system on a single silicon chip. These building blocks would typically include one or more CPU cores, RAM, ROM and peripheral and interface blocks, perhaps including blocks implementing a display interface, WiFi or Bluetooth.
Sod's Law	Murphy's Law states that if anything can possibly go wrong, it generally will, causing the maximum frustration and inconvenience. A well known example is that if you drop a slice of toast on the floor, it will nearly always fall butter side down. Sod's Law is a slightly more extreme formulation of the same phenomenon, stating that if something can go wrong, it will go wrong. Whereas Murphy's law is of US origin, Sod's Law is thought to have originated in the UK, possibly influenced by the generally inclement and miserable weather frequently suffered on this side of the Atlantic. Whichever of these is more accurate, they constitute absolute proof that we are living in a computer simulation with a bug in the software, since they are in direct contradiction to the laws of probability.
SODIMM	A SODIMM, or Small Outline Dual In-line Memory Module (sometimes known as a "RAM stick") is a small circuit board, one or more of which very commonly serve as main working memory in a laptop. It has contacts on both sides of an edge which mates with a socket (hence "Dual In-line") and contains a number of individual memory chips. There are different types containing various types and speeds of memory chip. A DIMM is a larger variant commonly used in destop computers.
Software	Software is the step-by-step instructions which tell a computer what to do and how to do it.
Software Defined Radio	An SDR or Software Defined Radio is a radio where the wanted signal is extracted by software, or in the case of a transmitter, the signal is generated by software. Thus, one radio can perform many different functions just by changing the software, whether it be sound or television broadcast, or data such as WiFi, Bluetooth or GPS. See Software Defined Radio in the Wiki.
SOGEA	SOGEA or Single Order Generic Ethernet Access is essentially the same as a FTTC Internet connection except that no old-fashioned anlogue telephone service is delivered over the copper segment from the street cabinet to the customer premises. This makes it cheaper. Voice calls can still be made, but they are carried digitally over the broadband connection.
SOHO	SOHO or Small Office, Home Office is used to describe devices such as printers and routers, intended for domestic or small office use, rather than in an enterprise or similar environment in which they would typically be managed centrally by an IT department.
SOIC	SOIC (Small Outline Integrated Circuit) and SOP (Small Outline Package) refer to packages for surface mount integrated circuits, significatly smaller than traditional through-hole packages. There are several variants, including mini-SOIC, SSOP (Shrink SOP - still smaller than SOP), TSOP (Thin SOP - thinner than SOP) and TSSOP (both thinner and smaller).
Solar Cell	A PV or Photo Voltaic Cell (commonly known as a solar cell) is a device for converting light into electricity.
Solder	Solder is a relatively low melting point mixture of (most often) tin and copper or tin and lead, making it very convenient for making electrical connections. (Americans call it "sodder", and many electrons have been harmed in discussions as to which is correct. But the Americans are on their own on this one.)
Solenoid	A solenoid is an inductor or electromagnet in cylindrical form.
Solid State Disk	A Solid State Disk (or SSD) is a faster (though more expensive) alternative to a hard disk for a computer's long term storage. Instead of a spinning magnetic disk, it stores data as trapped electrical charges in silicon chips. With no moving parts, it's much more robust than a hard disk. (It's a strange term in a way as it's no more solid than a hard disk.) See All about SSDs in the Wiki.
SOP	SOIC (Small Outline Integrated Circuit) and SOP (Small Outline Package) refer to packages for surface mount integrated circuits, significatly smaller than traditional through-hole packages. There are several variants, including mini-SOIC, SSOP (Shrink SOP - still smaller than SOP), TSOP (Thin SOP - thinner than SOP) and TSSOP (both thinner and smaller).
SOT	SOT (Small Outline Transistor) is a type of surface mount package used for semiconductor devices requiring only a few connections. SOT-23 is a 3-pin package only 2.9mm long, very commonly used for transistors, voltage regulators and diodes. Other variants have up to 8 pins or have a tab instead of pins on one side, to help conduct heat away.
Source	The source, gate and drain are the three terminals of a field-effect transistor (FET). See Transistors in the Wiki.
Source Code	Source code is software in a human-readable form as created by a developer. Generally, this cannot be directly understood or run by a computer, but has to be translated into object code by a special program called a compiler, or read, interpreted and executed line by line by a program known as an interpreter.
Spam	Spam is brand of tinned processed pork. With reference to a famous Monty Python sketch, the term has been adopted to refer to any unwanted email. Such emails may simply be an irritation, such as unwanted advertising, but often it will be a scam or contain malicious attachments or links.
Speculative execution	High performance CPUs use multiple logic blocks in order to perform as much work as possible in parallel. So for example, if a CPU is required to process a list of ten items it might start to "speculatively execute" the processing of an 11th before it has been able to determine that it has completed the list. Even though this wasted effort has to be discarded, a net performance improvement can be achieved.
Spear Phishing	Spear phishing is a type of phishing attack directed at a specific victim who has probably been researched in order to make the attack seem highly plausible.
SPDT	A SPDT (Single Pole Double Throw) switch has one contact (or pole) which can be connected to one of two other contacts, so completing one or other of two alternate circuits. It's often simply called a two-way switch.
SPI	SPI is a method of relatively high speed serial communication between ICs or subsystems in a device. It is used by SDCards and memory ICs more generally, as well as by displays and numerous other devices. It uses a clock line (SCLK) which simultaneously marches data in both directions using the two data lines MOSI (Master Out Slave In) and MISO (Master In Slave Out), making a minimum of 3 connections.
SPST	A SPST (Single Pole Single Throw) switch has one contact (or pole) which can be connected (or not) to one other contact to complete a circuit. In short, it's a simple on/off switch.
Spudger	A spudger is a tool with a thin plastic or metal blade typically used (for example) for inserting into the crack between two halves of a plastic case in order to pry it open.
Spyware	Spyware is a type of malware which reports browsing history and other personal information back to its creator without your permission.
SSD	A Solid State Disk (or SSD) is a faster (though more expensive) alternative to a hard disk for a computer's long term storage. Instead of a spinning magnetic disk, it stores data as trapped electrical charges in silicon chips. With no moving parts, it's much more robust than a hard disk. (It's a strange term in a way as it's no more solid than a hard disk.) See All about SSDs in the Wiki.
SSL	SSL or Secure Sockets Layer is a deprecated standard for securing data over a TCP data stream. It offers encryption using a shared key derived using a secure key exchange mechanism, and allows one or both ends to identify themselves using digital certificates. SSL went through several major versions, but 3.0 was deprecated in 2015 and has been superseded by TLS. Nevertheless, "SSL" is still often used to describe a link actually secured by TLS.
Stator	The stator of an electric motor is the part that doesn't move, as opposed to the rotor which is the part that rotates. More generally, a stator might be a fixed part of a machine which also contains moving parts.
Stepper Motor	A stepper motor is a type of motor which, instead of turning continuously, turns by a certain fraction of a revolution and stops every time it is given an electrical pulse. Used, for example, for stepping the paper through a printer as it is being printed. See DC and Universal (AC/DC) Motors in the Wiki.
Superhet	A superhet is a common design of radio receiver in which the incoming signal, whatever its frequency, is converted into a fixed intermediate frequency to make it easier to tune and amplify. See Superhet in the Wiki.
Supply chain attack	Modern hardware and software is generally composed of many different (hardware and software) components, some of these themselves composed of multiple sub-components. Keeping track of the provenance and trustworthiness of all of these can be an extremely difficult problem. Any of them might be compromised in a "supply chain" attack to compromise the integrity of a finished product. A classic example was the introduction of explosives into pagers and walkie-talkies supplied to Hezbollah in 2024, but purely software examples are increasingly a concern.
Surface Mount	Surface mount is a method of manufacture in which electronic components are soldered directly onto the surface of a printed circuit board, as opposed to the older through-hole method.
SWG	SWG or Standard Wire Gauge is a set of wire diameters where each gauge is approximately 10% thinner than the previous. For example 16 gauge is 1/16in in diameter and 50 gauge is a thousanth of an inch. Cross sectional area in square millimetres is becoming a more usual measure. See Standard Wire Gauge in Wikipedia.
Switch-mode Regulator	A switch-mode regulator is a circuit which produces a constant well defined voltage from a variable or poorly defined voltage input, by switching the power on and off very rapidly and smoothing the result. See Switching Mode Power Supplies in the Wiki.
Symmetric cryptography	In symmetric cryptography, decryption simply runs the encryption process backwards using the same key, unlike asymmetric or public key cryptography. AES is the commonest example. A problem is that for two parties to communicate securely they first need to securely agree a key. Public key cryptography overcomes this, but is computationally much more expensive.
Synchronous motor	A synchronous motor is a type of electric motor which only runs on AC, and which runs at a speed which is locked to the AC supply frequency. See Synchronous Motors in the Wiki.
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Tad	A tad is a measure of the carefully calibrated adjustment required to make something just spot-on. Most authorities consider it to be a little more than a smidgeon.
TB	A TB or Terabyte is 1000 gigabytes, but often used for 1024 gigabytes, which is more properly written TiB.
TCON	The TCON or Timing Controller board in a flat screen TV or monitor takes the video signal and converts it to a form the display panel can directly use, selecting and activating rows and columns of pixels in a timed sequence as required by the panel. A faulty TCON board is a possible cause of various types of display problem. See Flat screen TVs and Monitors in the wiki.
TCP	TCP or Transmission Control Protocol is an Internet protocol which provides a reliable bidirectional data stream between two Internet-connected devices. It uses IP to transmit the data as a series of packets, keeping track of them so as to request retransmission of any that get lost and re-ordering any that arrive out of sequence. It is used where it's vital that all the data arrives exactly as sent even though that might incur delays due to retransmissions.
TCXO	A Temperature Compensated Crystal Oscillator or TCXO comprises a quartz crystal and a control IC in a single package. A temperature sensor on the IC allows it to compensate for the variations in frequency of the crystal with temperature, resulting in considerably improved accuracy.
THD	THD or Total Harmonic Distortion is a measure of the degree to which an audio system distorts its input, effectively introducing new frequencies into the output.
Thermal Cut-out	A thermal fuse or thermal cut-out is a safety device which interrupts a circuit if the temperature rises above a certain critical level. Some are single use and must be replaced, whilst others reset automatically or can be reset manually. See Protection Components in the Wiki.
Thermal Fuse	A thermal fuse or thermal cut-out is a safety device which interrupts a circuit if the temperature rises above a certain critical level. Some are single use and must be replaced, whilst others reset automatically or can be reset manually. See Protection Components in the Wiki.
Thermal Paste	Thermal paste is a paste with a special formulation making it a good conductor of heat, used to improve the transfer of heat from an electronic component to a heat sink.
Thermal Runaway	Some electronic components and circuits take more current as they get hotter. Unchecked, this can lead to a positive feedback effect in which excessive heat causes still more heat to be generated, ending in a catastrophic failure and possibly a fire.
Thermionic Valve	A thermionic valve or vacuum tube is an amplifying device consisting of a glass tube containing several electrodes in a vacuum. An electrically heated cathode emits electrons which are attracted by an anode. Their flow is controlled by the voltage on a wire mesh grid in their path. Long since superseded by much smaller and less power-hungry transistors in all but a few niche applications.
Thermistor	A thermistor is an electronic component like a resistor but with a resistance which decreases substantially with increasing temperature. Often used for measuring or monitoring temperature, for example to warn of a dangerous temperature rise in a lithium battery.
Three Phase	Three phase is a system of power transmission in which three wires carry AC phased like three blades of a propeller. Consequently they balance out and there is no need for any wires to carry the return current. As a result, three wires can carry three times as much power as two wires in a conventional single phase circuit.
Through-hole	Through-hole is a method of manufacture in which electronic components are attached to a circuit board by passing their leads or pins through holes in the board and soldering them to the board on the other side, as opposed to the more modern surface mount technique which allows much higher component densities.
Thunderbolt	Thunderbolt is a high speed data interface mainly used for storage and video display devices.
Thyristor	A thyristor, SCR or Silicon Controlled Rectifier is an electronic component with 3 leads used as a switch. See Transistors in the Wiki.
TiB	A TiB or terabyte is 1,024GiB, or 1,099,511,627,776 bytes. It is as much as can be addressed with 40 bits.
TLS	TLS or Transport Layer Security is a standard for securing data over a TCP data stream. It offers encryption using a shared key derived using a secure key exchange mechanism, and allows one or both ends to identify themselves using digital certificates. It has superseded SSL, but "SSL" is still often used to describe a link actually secured by TLS.
Tolerance	Tolerance is an indication of how closely a device conforms to its specification. For example, a 100Ω resistor measured accurately is unlikely to read precisely 100Ω, but if it has a tolerance of 5% it will have a value between 95Ω and 105Ω.
TOR	TOR (The Onion Router) is a method of hiding your location as you browse the Internet. Your data is encrypted with several successive layers of encryption and then passed through a series of routers. Each router peels off one layer of encryption (like layers of an onion) using an encryption key known only to itself. Thus, each router only knows its predecessor and successor, and the final destination cannot determine the originator.
Torque	Torque is a twisting force, such as applied by a screwdriver to a screw, a spanner to a nut, or produced by the spindle of an electric motor. It is measured in Newton Metres (Nm), being the twisting force created by a linear force of 1 Newton (102 grams weight on the earth) applied to a lever of length 1m, or twice the force applied to a lever of half the length etc.
Torx	Torx is a type of security screw or screwdriver with a 6-pointed star shape. See Types of screw head and screwdriver in the Wiki.
TOSLINK	TOSLINK (from TOShiba LINK) is a method of connecting audio devices such as CD players and amplifiers using optical fibre and optical connectors. These are immune to interference and ground loops, unlike copper connections. Audio is transmitted digitally. S/PDIF is the standard which defines how it is encoded.
Transducer	A transducer is any device which transforms energy or a measurement from one form to another, e.g. sound to/from electricity (speakers and microphones), or temperature, light, force etc. to an electrical signal.
Transformer	A transformer consists of two (or more) coils of wire wound around a magnetic core, used to step a voltage up or down or provide isolation between two circuits. See Transformers in the Wiki.
Transistor	A transistor is an electronic component with 3 leads used to amplify a signal or switch a current on or off. See Transistors in the Wiki.
Tri-wing	Tri-wing is a type of screw and screwdriver a bit like cross-head but having 3 arms instead of 4. See Types of screw head and screwdriver in the Wiki.
Triac	A triac is a type of thyristor which, unlike the standard type, can be used on AC. Commonly used in dimmer switches.
TRF	A TRF or Tuned Radio Frequency radio receiver is one in which the signal from the aerial is tuned and amplified at its incoming frequency before demodulation to recover the audio (or other) transmitted material. If the radio is to be tunable, this means that several tuned circuits have to be tuned in step, a problem overcome in the superhet radio design.
TRIM	TRIM is a means by which an operating system can tell an SSD about files that have been deleted. This is important for performance reasons related to the way an SSD works. See SSD Migration and Troubleshooting in the wiki.
Trimpot	A trimpot is type of potentiometer intended for occasional adjustment only, usually requiring a screwdriver. Sometimes it will be set on manufacture or calibration then locked with some kind of sealant.
Triode	A triode is a device with three electrodes. Most commonly used to denote a type of valve.
Trojan	A trojan is a type of malware which hides inside legitimate software. Named after the Trojan horse used by the ancient Greeks to insinuate select warriors into the city of Troy and so defeat it.
Tuned Circuit	A tuned circuit comprises a capacitor and an inductor connected together. They resonate at a particular frequency as energy is continually and repeatedly passed from one to the other and back again.
Two Factor Authentication	Two Factor Authentication (2FA) and Multi Factor Authentication (MFA) are authentication schemes requiring a user to pass two or more tests, different in kind, in order to prove their identity. These are usually chosen from something you know (for example a password), something you have (a physical token) and something you are (a biometric such as a fingerprint or facial scan). Such a sytem is much harder to compromise than a simple password login.
Two Step Authentication	Two-step authentication is a kind of "poor man's Two factor Authentication" where the second factor is a one time code delivered to you by text message. Whilst considerably better than a password on its own, this is no longer greatly recommended on account of the relative ease by which a determined criminal could get a SIM card registered to your phone number and so get hold of the one time code. These "SIM swap" scams are endemic in some countries.
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UART/USART	A UART or Universal Asynchronous Receiver Transmitter is a device, often a built-in peripheral in a microcontroller for converting data between parallel and asynchronous serial formats. A USART also supports synchronous serial data.
UDP	UDP or User Datagram Protocol is an Internet protocol which uses IP to provide for rapid and low overhead transmission of packets of data from one Internet-connected device to another. It is used in applications such as audio or video streaming where the loss of an occasional packet would be much less noticeable than the stuttering that would result from possibly multiple retransmission attempts.
USB	USB or Universal Serial Bus is a method of connecting a wide range of peripheral devices to a computer, such as keyboards and mice, storage devices and many more, and is also frequently used simply to provide power for recharging smartphones, media players and cameras etc. Several revisions of the standard from USB 1.0 to USB 4 have successively increased its data transfer capability. There are several different types of USB connector in addition to the non-reversible rectangular one fitted to the majority of computers and memory sticks. See USB Connectors in the Wiki.
UEFI	UEFI or Unified Extensible Firmware Interface is a program built in to newer PCs (since around 2011) which controls the PC prior to booting and initiates the boot process. (Older computers used BIOS). See BIOS vs UEFI in the Wiki.
Unbrick	A device is said to be "bricked" if it has been rendered non-functional, perhaps by loading the wrong firmware. "Unbricking" is the (sometimes difficult) process of restoring it to normal operation.
Unicode	Unicode is a comprehensive character encoding standard that enables the consistent representation and handling of text from most of the world's writing systems. Unlike ASCII, which uses 7 bits and defines 128 characters, Unicode uses from 1 to 4 bytes to support over 143,000 characters, covering a wide array of scripts, symbols, and emojis. This standard ensures that text is displayed correctly across different platforms and languages, promoting universal data exchange and processing.
Universal Motor	An AC/DC or Universal motor is a type of electric motor that runs equally well on AC or DC. See DC and Universal (AC/DC) Motors in the Wiki.
Upcycling	Upcycling is the practice of taking disused or faulty items or their major components and reusing them in a different way to produce something of greater artistic or environmental value, at the same time saving them from recycling or landfill. However, convincing examples are hard to find, particularly at scale. The traditional Japanese art of kintsugi is an example of upcycling.
UPS	A UPS or Uninterruptible Power Supply is a back-up power supply that automatically cuts in on failure of the mains, allowing computers, telecommunications and possibly life-support equipment to continue operating. Normally relying on rechargeable batteries, it may only be able to provide power for long enough for orderly shutdown of computers, or in a critical application, for the start-up of a back-up diesel generator.
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Vacuum Tube	A thermionic valve or vacuum tube is an amplifying device consisting of a glass tube containing several electrodes in a vacuum. An electrically heated cathode emits electrons which are attracted by an anode. Their flow is controlled by the voltage on a wire mesh grid in their path. Long since superseded by much smaller and less power-hungry transistors in all but a few niche applications.
Valve	A thermionic valve or vacuum tube is an amplifying device consisting of a glass tube containing several electrodes in a vacuum. An electrically heated cathode emits electrons which are attracted by an anode. Their flow is controlled by the voltage on a wire mesh grid in their path. Long since superseded by much smaller and less power-hungry transistors in all but a few niche applications.
Varicap diode	A varicap diode is a type of diode which exploits the capacitance of a reverse-biased diode. This capacitance varies with the reverse voltage applied, making it useful (for example) for automatically adjusting the tuning to lock an FM radio onto a station.
Varistor	A Voltage Dependent Resistor or varistor is a device which has a very high resistance until a certain voltage is reached, whereupon its resistance falls sharply. Used for absorbing transient voltage surges in a mains supply.
VDR	A Voltage Dependent Resistor or varistor is a device which has a very high resistance until a certain voltage is reached, whereupon its resistance falls sharply. Used for absorbing transient voltage surges in a mains supply.
VGA	VGA (or Video Graphics Adapter) the the oldest, yet still very widely used standard for connecting a video monitor to a computer. It uses a 15 pin D-shaped connector carrying separate analogue signals representing the red, green and blue picture components.
Virtual Machine	A virtual machine or VM is a complete operating system with applications running in a kind of padded cell as an application under a host operating system. In this way, for example, you can try out different Linux distros under an existing Windows installation, or run a Windows installation under Linux if a Windows application you need is not available under Linux.
Virus	A virus is a term commonly used for any type of malware, but strictly meaning malware which self-replicates like a biological virus.
VM	A virtual machine or VM is a complete operating system with applications running in a kind of padded cell as an application under a host operating system. In this way, for example, you can try out different Linux distros under an existing Windows installation, or run a Windows installation under Linux if a Windows application you need is not available under Linux.
VoIP	VoIP or Voice over IP refers to the use of the Internet and Internet protocols (principally UDP) to implement traditional telephony in a much more flexible way. Your normal telephone handset can be replaced by an IP phone connected to your home router, or an app on your computer.
Volt	The Volt is the unit of electrical pressure, a bit like water pressure. Named after Italian physicist and chemist Alessandro Volta who is credited with the invention of the battery. See Electric circuits, volts amps watts and ohms in the Wiki.
Voltage Regulator	A voltage regulator is a circuit, often implemented as an integrated circuit, which delivers a constant well-defined output voltage from a poorly defined or variable input voltage (such as from a battery), despite variations (within certain limits) of the current drawn.
Vulnerability	A vulnerability is a flaw in a website or software product by which it can be compromised for malicious purposes, whatever they might be.
VSWR	If you connect a transmitter to an aerial via a long cable, at any discontinuity in the cable, some of the energy will be reflected back towards the transmitter instead of reaching the aerial. At some points between the transmitter and the discontinuity, the phases of the reflected and outgoing waves will reinforce and at other points they will partially cancel, creating "standing waves". The Voltage Standing Wave Ratio or VSWR is the ratio of the highest to the lowest voltages in the pattern of standing waves. Ideally, this ratio is 1:1, meaning that all the energy reaches the destination. The same principles apply to light encountering an imperfect splice or a poor termination in a fibre optic cable.
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WAF	The WAF or Wife Acceptance Factor is an assessment of the probability that your Significant Other will accept the installation of your latest techie toy in the living room without an irreversible breakdown in your relationship. (N.B. Restart does not endorse any stereotype implied by this term.)
Wall cube	A wall cube (also known as a wall wart) is that type of power supply which is built into a mains plug, for plugging directly into a wall socket. Chargers for phones and small gadgets most often come in this form.
Wall wart	A wall cube (also known as a wall wart) is that type of power supply which is built into a mains plug, for plugging directly into a wall socket. Chargers for phones and small gadgets most often come in this form.
WAN	A WAN or Wide Area Network is a computer network spanning a substantial geographical area.
Watering hole attack	A watering hole attack is an attempt to compromise a target organisation or individual by first compromising a website or online resource that they are known or suspected to use (their "watering hole"). In this way the attacker may hope to gain access to a well defended target, for example a defence contractor, though one of its less secure suppliers.
Watt	The Watt is the unit of power, calculated by multiplying the Volts by the Amps. Named after Scottish inventor, mechanical engineer and chemist James Watt, who invented the first truly practical steam engine. See Electric circuits, volts amps watts and ohms in the Wiki.
Waveguide	At the highest frequencies (GHz and above), every bit of wire acts like an aerial, so you can no longer rely on sending a signal down a wire and expecting it to get to the other end. So why not let it do its thing? Feed it into an aerial at the end of a copper pipe and it'll bounce all the way down the inside and come out at the far end. This is what a waveguide is. A microwave oven contains a waveguide to feed the energy from the magnetron into the cooking compartment.
Wavelength	Wavelength is the distance between successive crests of a radio wave as it travels through the air. If you divide the speed of light (300,000,000 m/s) by the wavelength you get the frequency, or by the frequency to get the wavelength. See Frequency vs wavelength in the Wiki.
Webmail	Webmail is the method of managing your email entirely through a web page offered by your email provider, who stores your email on their own servers. The alternative is to manage your email through an application on your own device, which can give you greater flexibility in organising, searching and archiving your email.
WEEE	The Waste Electrical and Electronic Equipment or WEEE Directive obligates producers and distributors of electrical equipment placed on the market after 2005 to make provision for its end of life collection and recycling. It is applicable in the UK and the EU. It has been criticised for its disregard for reuse or repair as a means of reducing e-waste.
WiFi	WiFi is a family of wireless local area networking protocols, allowing a number of computers and other devices within a reasonably confined space to talk to each other.
Worm	A worm is a type of malware able to spread from one computer to another across a network, or across the whole Internet.
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X.509	X.509 is a standard defining a format for a digital certificate, associating an identity such as the owner of a web site or email address or a software publisher and a public key. The certificate can then be used to cryptographically verify the authenticity of a web page, email or software purporting to come from the identity. The authenticity of the digital certificate itself is guaranteed by a chain of higher level digital certificates ending in a "root certificate" which everyone trusts.
x86	x86 is the Intel CPU architecture derived originally from the 8086 chip, released in 1978. Subsequent generations of chip were the 80286 (often referred to simply as 286), 386, 486 and 586 also known as Pentium. From 386 on, these were all 32 bit architectures, but since the mid to late 2000's the x86 architecture has been largely superseded by AMD64, though AMD64 processors retain the capability to run code written for x86.
XP	Windows XP is an obsolete Windows operating system first released in 2001. Extended support ended in 2014 and remaining users are strongly advised to migrate to a supported operating system as security patches are no longer provided.
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Yagi	Yagi is a type of aerial commonly used for terrestrial television reception, and consisting of an array of parallel rods, the array pointing in the direction of the transmitter.
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Zener diode	A zener diode is a type of diode which starts conducting in the reverse (normally non-conducting) direction at a well defined voltage, often used in electronic circuits to define a threshold or specific voltage.
Zero Day	A zero-day is a vulnerability that is found and already being exploited by malicious actors before any fix is available, giving potential victims no time at all (zero days) in which to protect themselves. Also known as an 0-Day (pronounced "oh-day").
ZIF Connector	A ZIF or Zero Insertion Force connector is one in which a clamp of some sort is closed to make a good contact after the mating parts have been engaged, instead of the force being applied during insertion. They are commonly used for delicate ribbon cables, and for test rigs where many insertions of devices under test will routinely be made.